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            The Digestive                                             CHAPTER OUTLINE
            System                                             13
           H        ave you seen Super Size Me—the
                    movie by the man who ate nothing but
           McDonald’s for one excruciating month? Part              ■ Nutrients Are Life-sustaining
           of the delicious delight of watching Morgan                p. 000

           Spurlock work his way through endless Big
           Macs stems from pure contrariness. Your
           mother, after all, told you not to eat junk food,
           and here is Spurlock, gobbling like mad. The
           other delight comes from mother’s
           vindication. Sure enough, Spurlock suffers
                                                                    ■ The Digestive System Processes
           mightily for his excess.
                                                                      Food from Start to Finish p. 000
                Long ago, when the Beatles sang, “You
           know that what you eat, you are,” the idea that
           food might affect health was revolutionary. But
           not anymore. Nowadays, the idea that the
           food that you consume can affect your health
           is commonplace, and indeed many are sur-
           prised by a study that finds, for example, that
                                                                    ■ Digestion Is Both Mechanical and
           eating less fat may not reduce the incidence of            Chemical p. 000
           breast cancer, or that calcium supplements
           may not ward off osteoporosis.
                At the center of all this concern is the
           digestive system, an essential series of
           organs that are designed to extract every
           last gram of nutrition from whatever goes
           down the gullet. In an era of rising obe-                ■ Nutritional Health and Eating
           sity, such efficiency is not necessarily a                 Disorders: You Truly Are What You
           good thing: Some designer fats are being                   Eat p. 000

           deliberately concocted to avoid diges-
           tion. But that’s the exception. In general,
           the goal of the digestive system is to
           convert food into simple compounds that
           the body can use for making adipose tissue,
           cellular energy, adenosine triphosphate (ATP),
           and the building blocks necessary for
           constructing cells and tissues.

           404
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                                                                                                                                           Glycolysis, the Krebs cycle, and
           Nutrients are Life-Sustaining                                                                                                   electron transport Figure 13.1
                                                                                                                                                                                                                1
                                                                                                                                                                                                                                    Glucose


                                                                                                                                                                                                              In cytosol   Glycolysis                  ATP




                                                                                                                                                                                                                                                                                        Process Diagram
                                                                          (vitamins and minerals). These are organic and inor-             Glycolysis occurs in the cytoplasm, requiring two molecules
              LEARNING OBJECTIVES
                                                                          ganic compounds, obtained from food rather than syn-             of ATP to begin, but generating a total of four ATP molecules
              Differentiate between macronutrients and micronutrients.                                                                     in the conversion of glucose to pyruvate. With oxygen
                                                                          thesized by us. We ingest carbohydrates, lipids, and pro-                                                                                                Pyruvic acid
              Describe how nutrients enter our cells.                                                                                      present, the two pyruvate molecules are shuttled to the
                                                                          teins to provide the necessar y energy and starting                                                                                                                                Mitochondrion
                                                                                                                                           mitochondrion, where they are passed through a series of            Mitochondrial
                                                                          materials for us to create our own carbohydrates, lipids,
                                                                                                                                           chemical reactions, each step of which releases energy that         matrix
                                                                          and proteins. From these macronutrients, we synthesize
                                                                                                                                           is harvested in ATP, NADH and FADH2. These reactions are
                                                                          cellular components such as the cell membrane, en-               referred to as the Krebs, or TCA, cycle. The NADH and
                             ll aerobic cells, and therefore all hu-


              A
                                                                          zymes, organelles, and even entirely new cells during                                                                                                                CO2
                             mans, need oxygen to survive. This oxy-                                                                       FADH2 created in the Krebs cycle then drive the reactions of
                                                                          mitosis and meiosis. Micronutrients are required for             the electron transport chain, where hydrogen ions are                                               NAD+
                             gen drives cellular respiration by ser v-                                                                                                                                                         2
                                                                          the proper functioning of essential compounds, such as           moved to the center of the mitochondrion, creating a
                             ing as the ultimate electron “pull,”                                                                                                                                                                               NADH + H+
                                                                          the enzymes of cellular respiration. Review Chapter 2,           hydrogen ion gradient. This gradient drives chemiosmosis,
           creating the hydrogen ion concentration gradient re-
                                                                          Everyday Chemistry, to refresh your understanding of             the final step in this process. At this point, the energy                           Acetyl Coenzyme A
           quired to form ATP. However, one cannot live by oxy-
                                                                          carbohydrates, lipids, and proteins.                             harvested from the original glucose molecule is finally
           gen alone!                                                                                                                      converted to 32 ATP molecules.
                     The cells of our body require nutrients in usable
           form to maintain homeostasis and create ATP. Because           THERE ARE THREE CLASSES                                          ● Glycolysis. Oxidation of one glucose molecule to two
                                                                                                                                           1                                                                               KREBS                  CO2
                                                                                                                                               pyruvic acid molecules yields 2 ATPs.                                       CYCLE
                                       we are heterotrophs, we cannot     OF MACRONUTRIENTS                                                                                                                         3
                                       manufacture our own organic                                                                         ● Formation of two molecules of acetyl coenzyme A yields
                                                                                                                                           2
                                                                                                                                                                                                                                              NADH + H+
              Aerobic                                                                                                                          another 6 ATPs in the electron transport chain.
                                       compounds and must obtain          The average supermarket contains more than 20,000
              Requiring oxygen
              to metabolize.
                                       them from the environment.         food products, but these all come down to three                  ● Krebs cycle. Oxidation of succinyl CoA to succinic acid
                                                                                                                                           3                                                                                                  FADH 2

                                       Consequently, we spend an aw-      macronutrient groups: carbohydrates, fats, and proteins.             yields 2 ATPs.

              Nutrients
                                       ful lot of our time locating,      These groupings are distinct from the six major food             ● Production of 6 NADH
                                                                                                                                           4                            6H yields 18 ATPs in the
                                       preparing, and ingesting food.     groups, which are classified by food type rather than                electron transport chain. Production of 2 FADH2 yields 4
              Ingredients in food
                                                Eating is so important    biochemical make-up. For example, fruits, a food group,              ATPs in the electron transport chain.
              that are required
                                       that virtually every culture has   provide us with carbohydrates in the form of fructose,                                                                              O2                    4
              by the body.
                                       elaborate rituals surrounding      and meats, another food group, are rich in protein.                                                                                                           Electron transport
                                                                                                                                                                                                                           ETC
                                       food. Think of your last                    The macronutrients we hear a lot about in diet                                                                                                       chain
           Thanksgiving celebration, or even your birthday. Both          discussions are carbohydrates, and for good reason.
                                                                                                                                       Glycolysis                 Chemiosmosis                                                          ATP
           of these events traditionally include a specific celebra-      They are our most efficient source of energy. Carbohy-
                                                                                                                                       The enzymatic              The diffusion of hydro-
           tory food: turkey with all the trimmings, or a cake with       drates are composed of carbon, hydrogen, and oxygen
                                                                                                                                       breakdown of               gen ions across a mem-
           candles. And in both cases, there were rituals surround-       in a 1:2:1 ratio. The most common carbohydrate, glu-                                                                                                          H2O
                                                                                                                                       glucose to pyruvate,       brane, generating ATP
           ing the food. We take a moment to reflect on all the           cose, has a chemical formula of C6H12O6. The cells of
                                                                                                                                       occuring within the        as they move from high
           good things in our life before eating Thanksgiving din-        our body are excellent at breaking down glucose to pro-                                                                                                                                            www.wiley.com/
                                                                                                                                                                                                                                                                             college/ireland
                                                                                                                                       cytoplasm.                 concentration to low.
           ner, and we sing “Happy Birthday” and blow out can-            duce ATP or to synthesize amino acids, glycogen, or
           dles before cutting into the cake.                             triglycerides. Carbohydrate digestion is so efficient that
                     Although we may not understand why, we in-           we can ingest glucose and break it down completely                     Carbohydrate digestion, or cellular respiration,          The mitochondrion completes the enzymatic burning
           nately know that we need nutrients in order to survive.        into energy, carbon dioxide, and water. Although we           is actually a controlled burning of the glucose molecule           of glucose by passing the compounds through first the
           But exactly what are nutrients?                                                        are efficient carbohydrate            through a series of enzymatic reactions. Burning re-               Krebs cycle, where energy-rich compounds are created,
           A nutrient is defined as any                                                           burning machines, sometimes           leases energy all at once, whereas carbohydrate metabo-            and then passing these energy-rich compounds
           compound required by the                                                               fad diets encourage us to avoid       lism releases that same energy gradually. The first reac-          through the electron transport chain. During these
           body. The two main types of nu-                                                        this energy source. The Health        tion is glycolysis, which converts one glucose molecule            steps, the carbon dioxide we exhale is produced.
           trients are macronutrients (car-                                                       Wellness and Disease box on           into two pyruvate molecules, releasing a bit of energy.            Chemiosmosis within the inner membrane of the mi-
           bohydrates, lipids, and pro-                                                           the Atkins diet takes a closer        Assuming oxygen is present, the pyruvates are then                 tochondrion produces most of the ATP for the cells
           teins) and micronutrients                                                              look at this.                         passed to a mitochondrion where oxidation continues.               (Figure 13.1).


           406     CHAPTER 13                                                                                                                                                                                                      Nutrients are Life-Sustaining             407
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                    Lipids—fats—are a second class of macronu-                           ture and are usually derived         Calories                           Vegan                          The last class of macro-                             MYPYRAMID IS A DIETARY GUIDELINE
           trient. Unlike carbohydrates, fats are long chains of                         from animals, but coconut oil        The amount of heat                A vegetarian who        nutrients is protein. Proteins
           carbon molecules, with many more carbon atoms and                             is also a saturated fat.             stored in food,                   consumes only           are an essential part of our                                 Food groups are not nutrient classes. Rather, food
           far fewer oxygen atoms than carbohydrates. We need a                                   The American Cancer         equal to the                      plant products,         daily diet because amino acids                               groups are the major categories of foods: meats, dairy,
           little fat in our diet; however, fats are added to many                       Society reports that diets high      amount of heat it                 eating no animal        are not stored in the body. In-                              breads and pastas, vegetables, and oils or fats. Each
           dishes in one form or another. They carry flavor and                          in fat can increase the inci-        takes to raise the                products                stead of completely breaking                                 group is important to overall health, and each group
           add texture to food. According to marketing tests,                            dence of cancer, and gives a         temperature of 1                  whatsoever.             down the amino acids of in-                                  has a different daily caloric intake recommendation.
           they coat our mouths and provide a much-craved oral                           number of recommendations            kilogram of water 1                                       gested proteins for energy, we                               For example, the recommended daily allowance (RDA)
           gratification. Fats can be either saturated, meaning                          for minimizing your risk             degree celsius.                                           usually recycle them into pro-                               for meats is quite low, at two ser vings per day, or 50
           the carbon chain has ever y space occupied with hy-                           (Table 13.1). They reason                                            teins of our own. Of the 20 amino acids that make up liv-                              grams for women and 63 for men. Most Americans get
           drogens, or unsaturated, meaning there are some dou-                          that these diets are high in                                         ing organisms, we can manufacture only 12. The remain-                                 far more than that in their diet.
           ble bonds in the carbon chain (Figure 13.2). Be-                              calories, leading to obesity. Obesity is in turn associ-             ing eight essential amino acids must come from our diet                                        You may be familiar with the traditional food
           cause double bonds kink the long carbon chains,                               ated with increased cancer risks. They note that satu-               (Table 13.2a). This presents a problem only for                                        guide pyramid, which suggests healthy proportions of
           unsaturated fats cannot pack tightly together. Unsatu-                        rated fats may increase cancer risk, whereas other fats,             those individuals who choose not to consume red meat.                                  the food groups, based on the eating habits of healthy
           rated fats, including vegetable oils, are liquid at room                      such as omega-3 fats from fish oils, may reduce the risk                      Complete proteins, such as red meat and fish,                                 people in the United States and around the world. The
           temperature. Saturated fats are solid at room tempera-                        of cancer.                                                           contain all 20 amino acids. Unlike meat, no single veg-                                pyramid offers guidelines on the number of servings of
                                                                                                                                                              etable or fruit contains all eight essential amino acids.                              each type of food that should be eaten each day. The
                                                                                                                                                              But for those who choose to restrict meat intake, eating                               bottom of the pyramid is breads, cereals, and pastas,
                                                                                                                                                              legumes and grains, or combining cereal with milk, will                                with a recommended 6 to 11 ser vings per day. Fruits
               H H H H H H H H H H H H H H H                         O                          Good and Bad Fats Table 13.1                                  provide a full complement of amino acids. Vegans and                                   and vegetables are next, with a recommended 3 to 5
            H C C C C C C C C C C C C C C C C                             Acid Group                                                                          vegetarians can be quite healthy, assuming they moni-
                                                                     OH                         To limit your intake of cholesterol, trans fat,                                                                                                      servings of each daily. Milk and cheeses, proteins and
               H H H H H H H H H H H H H H H
                                                                                                and saturated fat                                             tor their protein intake. See Table 13.2b for a list of                                beans both fill the next level at 2 to 3 servings of each a
            Saturated fatty acid: palmitic acid                                                  •   Trim the fat from your steak and roast beef              food combinations that contain complementary amino                                     day. The top of the pyramid is fats, with a recommenda-
                                                                                Carbon-carbon
                                                                                double bonds     •   Serve chicken and fish, but don’t eat the skin           acids.                                                                                 tion that their use be “sparing.”
               H H H H H H H H                     H H H H H H H           O                     •   Try a vegetarian meal once a week
            H C C C C C C C C C C C C C C C C C C
                                                                           OH
                                                                                                 •   Limit your eggs to once or twice a week
               H H H H H H H H H H H H H H H H H
                                                                                                 •   Choose low-fat milk and yogurt
            Monounsaturated fatty acid: oleic acid (omega-9)                                                                                                      Essential and nonessential amino acids                                               Complementary proteins
                                                                                                 •   Use half your usual amount of butter or margarine
                                                                                                 •   Have only a small order of fries or share them with          Table 13.2a                                                                          Table 13.2b
               H H H H H               H           H H H H H H H           O                         a friend
            H C C C C C C C C C C C C C C C C C C                                                                                                                 Essential Amino Acids                  Nonessential Amino Acids
                                                                           OH                                                                                                                                                                          Rice & beans
               H H H H H H H H H H H H H H H H H                                                                                                                  Histidine                              Alanine
            Polyunsaturated fatty acid: linoleic acid (omega-6)                                 To increase your intake of polyunsaturated and                                                                                                         Rice and lentils
                                                                                                                                                                  Isoleucine                             Arginine*
                                                                                                monounsaturated fats
                                                                                                 •   Use olive, peanut, or canola oil for cooking and salad       Leucine                                Asparagine                                    Bread with peanut butter
               H H           H         H           H H H H H H H           O                         dressing                                                     Lysine                                 Aspartic acid (aspartate)
            H C C C C C C C C C C C C C C C C C C                                                                                                                                                                                                      Tofu and cashew stirfly
                                                                           OH                    •   Use corn, sunflower, or safflower oil for baking             Methionine                             Cysteine (cystine)*
               H H H H H H H H H H H H H H H H H
                                                                                                 •   Snack on nuts and seeds                                                                                                                           Bean burrito in corn tortilla
            Polyunsaturated fatty acid: alpha-linoleic acid (omega-3)                                                                                             Phenylalanine                          Glutamic acid (glutamate)
                                                                                                 •   Add olives and avocados to your salad                                                                                                             Hummus/chick peas & sesame seeds
                                                                                                                                                                  Threonine                              Glutamine*
           Saturated and unsaturated fats                                                                                                                         Tryptophan                             Glycine*                                      Black-eyed peas and corn bread
           Figure 13.2                                                                          To up your omega-3 intake                                         Valine                                 Proline*
                                                                                                                                                                                                                                                       Tahini (sesame seeds) and peanut sauce
                                                                                                 •   Sprinkle flax seed on your cereal or yogurt                  missing ???????                        Serine
           Almost all animal fats are saturated fats, especially those found in
                                                                                                 •   Add another serving of fish to your weekly menu                                                                                                   Trail mix (soy beans and nuts)
           beef and dairy products. Most plants produce unsaturated fats,                                                                                         missing ???????                        Tyrosine*
           the notable exceptions being coconuts, cocoa butter, and palm kernel                  •   Have a leafy green vegetable with dinner
                                                                                                                                                                                                                                                       Rice an tofu
                                                                                                                                                              * These amino acids are considered conditionally essential by the Institute of Medi-
           oils. For this reason, vegetable oil is liquid at room temperature,                   •   Add walnuts to your cereal                               cine, Food and Nutrition Board (Dietary Reference Intakes for Energy, Carbohydrates,
                                                                                                                                                              Fiber, Fat, Protein and Amino Acids. Washington, DC: National Academy Press, 2002).
           whereas butter or cocoa butter is solid.



           408       CHAPTER 13 The Digestive System                                                                                                                                                                                                                             Nutrients are Life-Sustaining   409
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                                                                                                                                                                                         The U.S. Department of Agriculture recently          more personal, giving recommendations for ser ving
                                         Atkins diet: Will eliminating carbohydrates help me lose weight?                                                                       updated its food pyramid with MyPyramid, found on-            size and number based on age, gender, and activity
                                                                                                                                                                                line at http://www.mypyramid.gov (Figure 13.3).               level. When you submit your personal statistics to the
                                         In 1972, cardiologist Dr. Robert Atkins rocked the diet
   Health, Wellness, and Disease

                                                                                                         first study showed that its “successful losers” were eat-
                                                                                                                                                                                Although this pyramid is more in tune with current re-        MyPyramid Web site, you receive food intake guidelines
                                         world with his book on a “diet revolution” that placed ex-      ing a low-calorie, low-fat diet—the opposite of Atkins.
                                                                                                                                                                                search, it is based on the same principles as the tradi-      specific to your lifestyle. Underneath your MyPyramid
                                         treme emphasis on protein and fat, and discouraged eat-               Other concerns focused on safety. With heart dis-
                                                                                                                                                                                tional pyramid. It still recommends that we get most of       are a few suggestions for improving your choices within
                                         ing vegetables or carbohydrates. When a revised version         ease still the number 1 killer, did it make sense to pro-
                                                                                                                                                                                our caloric value from carbohydrates and that we limit        each group. The suggested amount of whole grains is
                                         of the diet was published in 1992, the book became an in-       mote eating fat, which gathers in the arteries and con-
                                         stant best-seller. Dieters waxed rhapsodic about the quick      tributes to atherosclerosis? With the antioxidants in                  our fat intake. Rather than arrange the food groups           listed as a portion of the carbohydrates, and the veg-
                                         and persistent weight loss they obtained by cutting carbs       vegetables playing an ever-clearer role in health, should              horizontally, however, they are arranged vertically. This     etable group is divided into dark greens, orange vegeta-
                                         and preferring protein.                                         dieters abandon the antioxidant-laden broccoli for high-               gives a more accurate visual picture because we require       bles, dry beans and peas, starchy vegetables, and others.
                                              The physiology is pretty simple. Lacking carbohy-          fat meat? Doctors also pointed to the known side effects               all the food groups in order to be healthy. We should         Although this is by no means an exhaustive view of
                                         drates, the normal source for glucose needed to produce         of a high-protein, high-fat diet, including kidney failure,            not base our caloric intake on carbohydrates, but we do       good eating, it does provide enough of a base for you
                                         ATP, the body mobilizes fat stores and converts fat into        high blood cholesterol, osteoporosis, kidney stones, and               get a majority of our calories from them. This site is also   to begin making healthier choices.
                                         small molecules called ketones. As ketones are oxidized         cancer. The word from established medical organizations
                                         to produce ATP, the body enters a metabolic state called        was unequivocal: “The American Heart Association does
                                         ketosis. The quick weight loss of the first week is caused      not recommend high-protein diets for weight loss.”                                                                                                  VITAMINS AND MINERALS
                                         by water loss, and that loss cannot be sustained. Starting            It’s hard to know whether the Atkins diet failed un-
                                                                                                                                                                                                                                                             ARE MICRONUTRIENTS
                                         the second week, weight loss slows drastically, because         der a shower of expert criticism, or through the simple
                                         the only way to lose weight is to expend more energy            fact that people could not stay with it. At any rate, Atkins
                                                                                                                                                                                                                                                             A healthy diet must include vitamins
                                         than we take in, and Atkins is a calorie-rich diet.             blazed bright and fizzled like a comet zooming across the
                                                                                                                                                                                                                                                             and minerals. Unlike macronutrients,
                                              As the Atkins diet sold millions of copies, it attracted   night sky. After selling millions of books, Atkins Nutrition-
                                         a storm of criticism from researchers and organizations         als, Inc., filed for bankruptcy in 2005.                                                                                                            these micronutrients are not broken
                                         concerned with nutri-                                                                                   But the death of the                                                                                        down, but instead are required for en-
                                         tion and obesity. For                                                                              Atkins diet did not mark                                                                                         zyme function or specific protein syn-
                                         starters, they wanted to                                                                           the death of the frenzy                                                                                          thesis. Vitamins are organic substances,
                                         see the evidence that                                                                              over being fat. The na-                                                                                          such as thiamine, riboflavin, and vita-
                                         the diet worked. Al-                                                                               tional obesity epidemic                                                                                          min A (Table 13.3). Minerals are in-
                                         though the Atkins orga-                                                                            continues, and it’s safe to                                                                                      organic substances such as calcium,
                                         nization offered anecdo-                                                                           predict that another                                                                                             zinc, and iodine (Table 13.4).
                                         tal evidence,                                                                                      quack diet cannot be far                                                                                                  A healthy diet with plenty of
                                         independent re-                                                                                    off. We can only hope
                                                                                                                                                                                                                                                             fruit and vegetables will give you most of
                                         searchers could not find                                                                           that your knowledge of
                                                                                                                                                                                                                                                             the necessar y vitamins and minerals.
                                         proof. For example, the                                                                            human biology will pro-
                                         U.S. National Institutes                                                                           tect you from getting
                                         of Health keeps track of                                                                           suckered by an unhealthy
                                                                                                                                                                                                                                                             My Pyramid Figure 13.3
                                         people who have suc-                                                                               diet. In health, as in jobs,
                                         cessfully kept off at                                                                              lovers, and promises in                                                                                          It is important to note that carbohydrates
                                         least 13.6 kg for five                                                                             general, the same rule                                                                                           remain our best source of energy.
                                         years on its National                                                                              applies: If it sounds too
                                         Weight Control Registry                                                                            good to be true, it proba-
                                         (NWCR). The Registry’s                                                                             bly is.




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                                   410     CHAPTER 13 The Digestive System                                                                                                                                                                                         Nutrients are Life-Sustaining          411
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              Vitamins Table 13.3
                                                                                                                       Deficiency Symptoms                                                                                                                                Deficiency Symptoms
              Vitamin                Comment and Source                      Functions                                 and Disorders                               Vitamin             Comment and Source                      Functions                                  and Disorders
              Fat–soluble            All require bile salts and some                                                                                               B2 (riboflavin)     Small amounts supplied by bacteria      Component of certain coenzymes             Deficiency may lead to improper uti-
                                     dietary lipids for adequate                                                                                                                       of GI tract. Dietary sources include    (for example, FAD and FMN) in              lization of oxygen resulting in blurred
                                     absorption.                                                                                                                                       yeast, liver, beef, veal, lamb, eggs,   carbohydrate and protein metabo-           vision, cataracts, and corneal ulcera-
                                                                                                                                                                                       whole-grain products, asparagus,        lism, especially in cells of eye, in-      tions. Also dermatitis and cracking of
              A                      Formed from provitamin                  Maintains general health and vigor        Deficiency results in atrophy and kera-                         peas, beets, and peanuts.               tegument, mucosa of intestine, and         skin, lesions of intestinal mucosa, and
                                     beta-carotene (and other provita-       of epithelial cells. Beta-carotene acts   tinization of epithelium, leading to dry                                                                blood.                                     one type of anemia.
                                     mins) in GI tract. Stored in liver.     as an antioxidant to inactivate free      skin and hair; increased incidence of
                                     Sources of carotene and other           radicals. Essential for formation of      ear, sinus, respiratory, urinary, and di-   Niacin              Derived from amino acid trypto-         Essential component of NAD and             Principal deficiency is pellagra, charac-
                                     provitamins include orange, yellow,     light-sensitive pigments in photore-      gestive system infections; inability to     (nicotinamide)      phan. Sources include yeast, meats,     NADP, coenzymes in oxidation-              terized by dermatitis, diarrhea, and
                                     and green vegetables; sources of vit-   ceptors of retina. Aids in growth of      gain weight; drying of cornea; and skin                         liver, fish, whole-grain products,      reduction reactions. In lipid metab-       psychological disturbances.
                                     amin A include liver and milk.          bones and teeth by helping to regu-       sores. Night blindness or decreased                             peas, beans, and nuts.                  olism, inhibits production of choles-
                                                                             late activity of osteoblasts and osteo-   ability for dark adaptation. Slow and                                                                   terol and assists in triglyceride
                                                                             clasts.                                   faulty development of bones and teeth.                                                                  breakdown.

              D                      Sunlight converts 7-dehydrocholes-      Essential for absorption of calcium       Defective utilization of calcium by         B6 (pyridoxine)     Synthesized by bacteria of GI tract.    Essential coenzyme for normal              Most common deficiency symptom is
                                     terol in the skin to cholecalciferol    and phosphorus from GI tract.             bones leads to rickets in children and                          Stored in liver, muscle, and brain.     amino acid metabolism. Assists pro-        dermatitis of eyes, nose, and mouth.
                                     (vitamin D3). A liver enzyme then       Works with parathyroid hormone            osteomalacia in adults. Possible loss of                        Other sources include salmon,           duction of circulating antibodies.         Other symptoms are retarded growth
                                     converts cholecalciferol to             (PTH) to maintain Ca2                     muscle tone.                                                    yeast, tomatoes, yellow corn,           May function as coenzyme in triglyc-       and nausea.
                                     25-hydroxycholecalciferol. A second     homeostasis.                                                                                              spinach, whole grain products, liver,   eride metabolism.
                                     enzyme in the kidneys converts                                                                                                                    and yogurt.
                                     25-hydroxycholecalciferol to cal-
                                     citriol (1,25-dihydroxycalciferol),                                                                                           B12                 Only B vitamin not found in vegeta-     Coenzyme necessary for red blood           Pernicious anemia, neuropsychiatric
                                     which is the active form of vitamin                                                                                           (cyanocobalamin)    bles; only vitamin containing cobalt.   cell formation, formation of the           abnormalities (ataxia, memory loss,
                                     D. Most is excreted in bile. Dietary                                                                                                              Absorption from GI tract depends        amino acid methionine, entrance of         weakness, personality and mood
                                     sources include fish-liver oils, egg                                                                                                              on intrinsic factor secreted by gas-    some amino acids into Krebs cycle,         changes, and abnormal sensations),
                                     yolk, and fortified milk.                                                                                                                         tric mucosa. Sources include liver,     and manufacture of choline (used           and impaired activity of osteoblasts.
                                                                                                                                                                                       kidney, milk, eggs, cheese, and         to synthesize acetylcholine).
              E (tocopherols)        Stored in liver, adipose tissue, and    Inhibits catabolism of certain fatty      May cause oxidation of monounsatu-                              meat.
                                     muscles. Sources include fresh nuts     acids that help form cell structures,     rated fats, resulting in abnormal struc-
                                     and wheat germ, seed oils, and          especially membranes. Involved in         ture and function of mitochondria,          Pantothenic acid    Some produced by bacteria of GI         Constituent of coenzyme A, which is        Fatigue, muscle spasms, insufficient
                                     green leafy vegetables.                 formation of DNA, RNA, and red            lysosomes, and plasma membranes.                                tract. Stored primarily in liver and    essential for transfer of acetyl group     production of adrenal steroid hor-
                                                                             blood cells. May promote wound            A possible consequence is hemolytic                             kidneys. Other sources include kid-     from pyruvic acid into the Krebs cy-       mones, vomiting, and insomnia.
                                                                             healing, contribute to the normal         anemia.                                                         ney, liver, yeast, green vegetables,    cle, conversion of lipids and amino
                                                                             structure and functioning of the                                                                          and cereal.                             acids into glucose, and synthesis of
                                                                             nervous system, and prevent scar-                                                                                                                 cholesterol and steroid hormones.
                                                                             ring. May help protect liver from
                                                                             toxic chemicals such as carbon                                                        Folic acid          Synthesized by bacteria of GI tract.    Component of enzyme systems syn-           Production of abnormally large red
                                                                             tetrachloride. Acts as an antioxidant                                                 (folate, folacin)   Dietary sources include green leafy     thesizing nitrogenous bases of DNA         blood cells (macrocytic anemia).
                                                                             to inactivate free radicals.                                                                              vegetables, broccoli, asparagus,        and RNA. Essential for normal pro-         Higher risk of neural tube defects in
                                                                                                                                                                                       breads, dried beans, and citrus         duction of red and white blood             babies born to folate-deficient
              K                      Produced by intestinal bacteria.        Coenzyme essential for synthesis of       Delayed clotting time results in exces-                         fruits.                                 cells.                                     mothers.
                                     Stored in liver and spleen. Dietary     several clotting factors by liver, in-    sive bleeding.
                                     sources include spinach, cauli-         cluding prothrombin.                                                                  Biotin              Synthesized by bacteria of GI tract.    Essential coenzyme for conversion          Mental depression, muscular pain, der-
                                     flower, cabbage, and liver.                                                                                                                       Dietary sources include yeast, liver,   of pyruvic acid to oxaloacetic acid        matitis, fatigue, and nausea.
                                                                                                                                                                                       egg yolk, and kidneys.                  and synthesis of fatty acids and
              Water–soluble          Dissolved in body fluids. Most are                                                                                                                                                        purines.
                                     not stored in body. Excess intake is
                                     eliminated in urine.                                                                                                          C (ascorbic acid)   Rapidly destroyed by heat. Some         Promotes protein synthesis includ-         Scurvy; anemia; many symptoms re-
                                                                                                                                                                                       stored in glandular tissue and          ing laying down of collagen in for-        lated to poor collagen formation, in-
              B1 (thiamine)          Rapidly destroyed by heat. Sources      Acts as coenzyme for many different       Improper carbohydrate metabolism                                plasma. Sources include citrus          mation of connective tissue. As            cluding tender swollen gums, loosen-
                                     include whole-grain products, eggs,     enzymes that break carbon-to-             leads to buildup of pyruvic and lactic                          fruits, tomatoes, and green             coenzyme, may combine with poi-            ing of teeth (alveolar processes also
                                     pork, nuts, liver, and yeast.           carbon bonds and are involved in          acids and insufficient production of                            vegetables.                             sons, rendering them harmless until        deteriorate), poor wound healing,
                                                                             carbohydrate metabolism of pyruvic        ATP for muscle and nerve cells. Defi-                                                                   excreted. Works with antibodies,           bleeding (vessel walls are fragile be-
                                                                             acid to CO2 and H2O. Essential for        ciency leads to: (1) beriberi, partial                                                                  promotes wound healing, and func-          cause of connective tissue degenera-
                                                                             synthesis of the neurotransmitter         paralysis of smooth muscle of GI tract,                                                                 tions as an antioxidant.                   tion), and retardation of growth.
                                                                             acetylcholine.                            causing digestive disturbances; skeletal
                                                                                                                       muscle paralysis; and atrophy of limbs;
                                                                                                                       (2) polyneuritis, due to degeneration
                                                                                                                       of myelin sheaths; impaired reflexes,
                                                                                                                       impaired sense of touch, stunted
                                                                                                                       growth in children, and poor appetite.




           412     CHAPTER 13 The Digestive System                                                                                                                                                                                                                      Nutrients are Life-Sustaining              413
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              Minerals Table 13.4
              Mineral                   Comments                                                  Importance                                                   Mineral                  Comments                                                Importance
              Calcium                   Most abundant mineral in body. Appears in combina-        Formation of bones and teeth, blood clotting, normal         Iodide                   Essential component of thyroid hormones. Excreted       Required by thyroid gland to synthesize thyroid hor-
                                        tion with phosphates. About 99% is stored in bone         muscle and nerve activity, endocytosis and exocytosis,                                in urine. Sources are seafood, iodized salt, and veg-   mones, which regulate metabolic rate.
                                        and teeth. Blood Ca2 level is controlled by parathy-      cellular motility, chromosome movement during cell                                    etables grown in iodine-rich soils.
                                        roid hormone (PTH). Calcitriol promotes absorption        division, glycogen metabolism, and release of neuro-
                                        of dietary calcium. Excess is excreted in feces and       transmitters and hormones.                                   Manganese                Some stored in liver and spleen. Most excreted in       Activates several enzymes. Needed for hemoglobin
                                        urine. Sources are milk, egg yolk, shellfish, and leafy                                                                                         feces.                                                  synthesis, urea formation, growth, reproduction, lacta-
                                        green vegetables.                                                                                                                                                                                       tion, bone formation, and possibly production and re-
                                                                                                                                                                                                                                                lease of insulin, and inhibition of cell damage.
              Phosphorus                About 80% is found in bones and teeth as phosphate        Formation of bones and teeth. Phosphates (H2PO4 ,
                                        salts. Blood phosphate level is controlled by parathy-    HPO4 , and PO43 ) constitute a major buffer system           Copper                   Some stored in liver and spleen. Most excreted in fe-   Required with iron for synthesis of hemoglobin. Com-
                                        roid hormone (PTH). Excess is excreted in urine;          of blood. Plays important role in muscle contraction                                  ces. Sources include eggs, whole-wheat flour, beans,    ponent of coenzymes in electron transport chain and
                                        small amount is eliminated in feces. Sources are dairy    and nerve activity. Component of many enzymes. In-                                    beets, liver, fish, spinach, and asparagus.             enzyme necessary for melanin formation.
                                        products, meat, fish, poultry, and nuts.                  volved in energy transfer (ATP). Component of DNA
                                                                                                  and RNA.                                                     Cobalt                   Constituent of vitamin B12.                             As part of vitamin B12, required for erythropoiesis.

              Potassium                 Major cation (K ) in intracellular fluid. Excess ex-      Needed for generation and conduction of action po-
                                        creted in urine. Present in most foods (meats, fish,      tentials in neurons and muscle fibers.                       Zinc                     Important component of certain enzymes. Wide-           As a component of carbonic anhydrase, important in
                                        poultry, fruits, and nuts).                                                                                                                     spread in many foods, especially meats.                 carbon dioxide metabolism. Necessary for normal
                                                                                                                                                                                                                                                growth and wound healing, normal taste sensations
              Sulfur                    Component of many proteins (such as insulin and           As component of hormones and vitamins, regulates                                                                                              and appetite, and normal sperm counts in males. As a
                                        chrondroitin sulfate), electron carriers in electron      various body activities. Needed for ATP production by                                                                                         component of peptidases, it is involved in protein
                                        transport chain, and some vitamins (thiamine and bi-      electron transport chain.                                                                                                                     digestion.
                                        otin). Excreted in urine. Sources include beef, liver,
                                        lamb, fish, poultry, eggs, cheese, and beans.                                                                          Fluoride                 Components of bones, teeth, other tissues.              Appears to improve tooth structure and inhibit tooth
                                                                                                                                                                                                                                                decay.
              Sodium                    Most abundant cation (Na ) in extracellular fluids;       Strongly affects distribution of water through osmosis.
                                        some found in bones. Excreted in urine and perspira-      Part of bicarbonate buffer system. Functions in nerve        Selenium                 Important component of certain enzymes. Found in        Needed for synthesis of thyroid hormones, sperm
                                        tion. Normal intake of NaCl (table salt) supplies         and muscle action potential conduction.                                               seafood, meat, chicken, tomatoes, egg yolk, milk,       motility, and proper functioning of the immune sys-
                                        more than the required amounts.                                                                                                                 mushrooms, and garlic, and cereal grains grown in       tem. Also functions as an antioxidant. Prevents chro-
                                                                                                                                                                                        selenium-rich soil.                                     mosome breakage and may play a role in preventing
              Chloride                  Major anion (CI ) in extracellular fluid. Excess ex-      Plays role in acid–base balance of blood, water bal-                                                                                          certain birth defects, miscarriage, prostate cancer, and
                                        creted in urine. Sources include table salt (NaCl), soy   ance, and formation of HCI in stomach.                                                                                                        coronary artery disease.
                                        sauce, and processed foods.
                                                                                                                                                               Chromium                 Found in high concentrations in brewer’s yeast. Also    Needed for normal activity of insulin in carbohydrate
              Magnesium                 Important cation (Mg2 ) in intracellular fluid. Ex-       Required for normal functioning of muscle and ner-                                    found in wine and some brands of beer.                  and lipid metabolism.
                                        creted in urine and feces. Widespread in various          vous tissue. Participates in bone formation. Con-
                                        foods, such as green leafy vegetables, seafood, and       stituent of many coenzymes.
                                        whole-grain cereals.

              Iron                      About 66% found in hemoglobin of blood. Normal            As component of hemoglobin, reversibly binds O2.
                                        losses of iron occur by shedding of hair, epithelial      Component of cytochromes involved in electron trans-
                                        cells, and mucosal cells, and in sweat, urine, feces,     port chain.
                                        bile, and blood lost during menstruation. Sources are
                                        meat, liver, shellfish, egg yolk, beans, legumes, dried
                                        fruits, nuts, and cereals.
                                                                                                                                                            Milled
                                                                                                                                                            Grain ground into
           However, many Americans now supplement their diets                       dine, magnesium, and zinc, among many other mi-                         flour.                Milled grains lose their fibrous,                  else. These are sometimes called “empty calories” on
           with moderate levels of vitamins and minerals, just to                   cronutrients. Some minerals are found in high concen-                                         mineral-rich outer husk, dimin-                    the theor y that they contribute more to weight gain
           ensure they receive what they need on a daily basis. The                 tration in foods, especially prepared foods. Sodium, for                ishing their nutritional value. Simple carbohydrates,                    than to homeostasis.
           usual supplement taken is an over-the-counter (OTC)                      example, is extremely high in most frozen and pre-                      such as sucrose, usually provide energy and nothing
           multivitamin supplement. These often include vitamins                    pared foods. Because a large quantity of these conve-
           E, C, and A, which help remove free radicals, thereby                    nience foods is consumed by the general population,
           boosting the immune system and perhaps prolonging                        sodium supplements are seldom advisable, because too                       CONCEPT CHECK
           cell life. As with anything, excess is not healthy. Taking               much sodium in the diet can lead to hypertension.
           too large a quantity of fat-soluble vitamins can cause                            By eating mostly whole grains, we obtain vita-                      What are the major           Describe the                    Differentiate
           them to build up in the liver, hampering its function.                   mins and minerals as well as glucose. Whole grain also                            macronutrients?             differences between             between vitamins
                     Selected minerals are usually also found in                    provides fiber, which helps move feces along the large                                                        the traditional food            and minerals.
           OTC multivitamins, such as calcium, phosphorus, io-                      intestine and decreases the risk of colon cancer.                                                             pyramid and
                                                                                                                                                                 What is a micronutrient?         MyPyramid.

           414       CHAPTER 13 The Digestive System
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           The Digestive System Processes Food                                                                                                                                                                                                                                                Mesentery



           From Start to Finish
                                                                                                                                                                                                                          Gland in                       Vein
                                                                                                                                                                                                                          mucosa
              LEARNING OBJECTIVES
                                                                                                                                                                                              Duct of gland                           Glands in
              Describe the general anatomy of the digestive tract.       Briefly explain the function of each organ in the digestive                                                          outside tract                           submucosa
                                                                         system.                                                                                                              (such as
              List the digestive organs in order from mouth to anus.                                                                                                                          pancreas)

                                                                                                                                                                                                                                                                                Artery

                                                                                                                                                                                                                                                                                Nerve
           THE GI TRACT REMAINS THE SAME
           THROUGHOUT ITS LENGTH                                                                                                       Mouth (oral cavity)
                                                                                                                                       contains teeth
                                                                                                                                                                                Lumen
                                                                                                                                       and tongue
           The digestive system is sometimes called a “tube within       Parotid gland
                                                                                                                                       Sublingual gland
           a tube,” because it is a hollow structure with two open-      (salivary gland)
                                                                                                                                       (salivary gland)
           ings that runs the length of your body. The digestive         Submandibular gland                                           Pharynx
                                                                         (salivary gland)
           system, also called the “gastrointestinal system” or GI                                                                                                              SUBMUCOSA
                                                                         Esophagus
           tract, begins at the oral cavity, winds through the ab-
           dominal cavity, and ends at the anus (Figure 13.4).                                                                                                                               MUSCULARIS:
                                                                                                                                                                                              Circular muscle
                    The structure of the GI tract is essentially the                                                                                                                          Longitudinal muscle
           same along its entire length. The innermost layer is
           composed of a mucous membrane, or mucosa. This                                                                                                                         Layers of the GI tract Figure 13.5
           slipper y, smooth layer allows ingested food to move
                                                                         Liver                                                                                                  The serosa allows the GI tract to move as food passes within it. The muscularis is responsible for generating
           along the tract without tearing it. Under the mucosa,                                                                                   Stomach
                                                                                                                                                                                the movement of the tube, whereas the mucosa and submucosa come into contact with the food and provide
           the submucosa includes the glands, nerves, and blood          Duodenum
                                                                                                                                                   Pancreas                     the blood supply and innervation for the inner lining of the tract.
           supply for the tract itself. The mucularis gives the tract    Gallbladder
           the ability to move substances lengthwise. For most of                                                                                  Transverse
                                                                         Jejunum                                                                   colon
           the tract, the mucularis is composed of one layer of lon-                                                                               Descending
                                                                         Ileum
           gitudinal muscle above another layer of circular muscle                                                                                 colon
                                                                         Ascending colon                                                                                              Esophagus
                                      ( Figure 13.5). These lay-                                                                                   Sigmoid colon                                                                       Peristaltic wave generation Figure 13.6
                                                                         Cecum                                                                                                        Relaxed
                                      ers work in unison to create                                                                                 Rectum
              Peristaltic wave                                                                                                                                                        muscularis
                                                                                                                                                                                                                                     The peristaltic wave is generated as you consciously swallow food.
                                      the peristaltic wave ( Fig-        Appendix
                                                                                                                                                                                      Circular muscles
              Rhythmic muscular                                                                                                                    Anus                               contract                                       Movement of the tongue initiates the muscularis to begin a ring of
                                      ure 13.6) that propels food
              contractions of a                                                                                                                                                                                                      contraction that is passed throughout the entire tract. Once you
                                      through the tube.
              tube that force                                                                                                                                                                                                        swallow food, the peristaltic wave travels the length of the tube;
                                              The outer layer of the                                                                                                                                              Lower
              contents toward                                                                                                                                                    Longitudinal muscles             esophageal         you no longer have conscious control over those smooth
                                      GI tract, the serosa, is a slip-                                                                                                           contract                         sphincter          mucle contractions.
              the open end.                                                                                                                                                     Relaxed muscularis
                                      per y membrane that permits                Right lateral view of head and neck and anterior view of trunk
                                      the tract to move inside the
                                                                           Digestive system overview Figure 13.4                                                   Bolus
           abdominal cavity without catching or causing discom-
           fort. Your digestive system is always active, as muscular     The tubular structure of the GI tract is obvious when looking
           contractions shift, lengthen, and shorten the tube. De-       at it in its entirety. The tube begins at the esophagus, and
                                                                                                                                                                   Stomach
           spite this constant movement, you normally neither see        with slight modifications, travels the length of the tract,
           nor feel the movement.                                        ending at the anus. These modifications alter the function
                                                                         of the tract at various points, which we describe as different
                                                                         organs.                                                                                           Anterior view of frontal sections of peristalsis in esophagus

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           416      CHAPTER 13 The Digestive System                                                                                                                                                                                        The Digestive System Processes Food From Start to Finish       417
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                                                                                                               Macerated                                                                                               Superior lip (pulled upward)
           DIGESTION BEGINS                                              We first obtain 20 primary, de-
                                                                                                               Soaked until soft
           IN THE ORAL CAVITY                                            ciduous, or baby teeth ( Fig-
                                                                                                               and separated into
                                                                         ure 13.7). These are re-
                                                                                                               constituent parts.                                                                                      Gingivae (gums)
           The best way to understand the actions of the digestive       placed by our 32 permanent
           system is to follow some food through the GI tract, start-    teeth, usually by age 21 (Fig-
                                                                         ure 13.8).                            Mechanical
           ing at the oral cavity, or mouth. Think about a hot slice                                                                     Hard palate
                                                                                   The small bits of pizza     digestion
           of pizza. How does it provide energy and nutrients?
                                                                                                               Physically crushing,      Soft palate
           Let’s follow that slice along the digestive tract, and see    are macerated with saliva.
                                                                                                               chopping, and
           how the body pulls nutrients from it, and how its en-         Mechanical digestion in-
                                                                                                               cutting food.             Uvula
           ergy is used to create adipose tissue for energy storage,     creases the efficiency of en-                                                                                                                 Palatine tonsil

           or ATP for immediate use.                                     zymes in the stomach and                                        Cheek
                                                                                                                                                                                                                       Tongue
                    The pizza enters the digestive tract through the     small intestine, by creating          Chemical
           oral cavity. We tear off a bite of pizza with incisors, and   small bits with a great deal of       digestion                 Molars                                                                        Lingual frenulum
                                                                         surface area where enzymes            Breaking down                                                                                           Opening of duct of
           then crush it with the molars and premolars. Teeth                                                                                                                                                          submandibular gland
                                                                         can carr y out the process of         food using                Premolars
           function as cutting tools (incisors), piercing and rip-
                                                                         chemical digestion.                   enzymes that alter
           ping utensils (canines), or grinding instruments (mo-                                                                         Cuspid (canine)                                                               Gingivae (gums)
                                                                                   Most people tr y to         the chemical
           lars and premolars). Although we are not born with                                                                            Incisors
                                                                         take good care of their teeth,        structure of
           teeth extending through the gums, they erupt soon af-
                                                                         with regular brushing, floss-         the food.
           ter birth in a predictable pattern. Incisors appear first,                                                                                                                                                  Inferior lip (pulled down)
           allowing food to be bitten off, often by 8 months of age.     ing, and visits to the dentist.
                                                                                                                                                                                 Anterior view
           The premolars and molars appear last, with “wisdom            Why do we bother with such
           teeth,” our final set of grinding molars, appearing           dental cleanliness? Your mouth contains hundreds of               Oral cavity Figure 13.8                                                                                    Bolus
           sometimes as late as our mid-twenties or early thirties.      species of bacteria, which live on the oral surfaces and                                                                                                                     A round, soft mass
                                                                                                                                         The teeth and tongue in the oral cavity are ideal for mechanical digestion. The food is rolled
                                                                         multiply rapidly when sugar is available. These bacteria                                                                                                                     of chewed food
                                                                                                                                         around with the tongue and broken into smaller pieces with the teeth.
                                                                         excrete wastes as they grow and metabolize. The wastes                                                                                                                       within the digestive
                                                                         are usually acidic, and if the acid remains on tooth sur-                                                                                                                    tract.
                                                                         faces, they can eat through the enamel to the softer
                                                                         dentin at the center of the tooth. Plaque is a combina-         called the pulp. By this time, the cavity is quite large              any direction in the oral cav-
                                                                                                                                                                                                                                                   Papilla
                                                                         tion of the bacterial colonies, their bacterial wastes, left-   and will require dental repair.                                       ity. On its surface, keratinized
                                                                                                                                                                                                                                                   Any small rounded
                                                                         over sugars from chewed up food, epithelial cells from                  The recommended biannual dental cleaning is                   epithelium covers papilla, cre-
                                                                                                                                                                                                                                                   projection
                                                                         the host, and saliva. Plaque begins as a sticky substance       a great way to monitor plaque buildup and cavity for-                 ating a rough texture to help
                                                                                                                                                                                                                                                   extending above
                                                                         on the surfaces of the teeth but can calcify with time          mation. While removing plaque, the hygienist may spot                 move the slipper y food into
                                                                                                                                                                                                                                                   a surface.
                                                                         into the tough layer of tartar your hygienist must              any small cavities, which the dentist can repair before               position where the teeth can
                                                                         scrape off.                                                     they destroy the pulp of the tooth. The repair process                masticate it. Taste buds reside     Lingual
                                                                                   The largest increase in bacterial growth occurs       involves drilling out all rotten enamel, and replacing it             along the sides of these            Relating to speech
                                                                         20 minutes after eating. The bacterial colonies are me-         with an air-tight seal made of gold, silver alloy, or com-            papilla. The tongue also se-        or the tongue.
                                                                         tabolizing the food from you last meal, growing and di-         posite resin. Mercury amalgam is no longer used to fill               cretes watery mucus contain-
                                                                         viding at their highest rate. As the bacteria are multiply-     cavities due to the health risks of mercury, which is a               ing a digestive enzyme, lingual
                                                                         ing rapidly, they are digesting the sugar in your mouth         potent neurotoxin. Some dentists recommend replac-                    lipase, from sublingual salivary glands on its undersur-
                                                                         and creating large quantities of acidic waste. Once the         ing old amalgam fillings with composite resin, to avoid               face. This enzyme begins the chemical digestion of
             The transition from baby teeth to                           food is removed, the bacterial division slows. If you do        later complications.                                                  lipids by breaking down triglycerides, such as those in
             permanent teeth Figure 13.7                                 not thoroughly and routinely remove this buildup of                                                                                   the pizza’s cheese.
                                                                         bacteria and acid, the acid may decay the enamel on             The tongue balls things up The tongue manip-
           Teeth erupt from the gums in a specific order as we mature.
                                                                         the teeth, causing cavities. A cavity does not cause pain       ulates the now-crushed pizza into a bolus and positions               The tonsils are the first line of defense
           They may appear more slowly in some individuals, but
           the pattern of eruption is predictable.
                                                                         at first, but as the acids reach farther into the tooth,        that bolus at the back of the oral cavity so it can be swal-          against microbes The uvula hangs from the top
                                                                         they eventually hit softer tissue near the tooth’s nerve,       lowed. The tongue is a muscle that can move in almost                 of the oral cavity at the back of the mouth. This struc-


           418     CHAPTER 13 The Digestive System                                                                                                                                                   The Digestive System Processes Food From Start to Finish           419
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           ture functions as a trap door, swinging upward and clos-      (sodium, potassium, chloride, bicarbonate, and phos-               person to person in saliva, either by inhaling small bits              Deglutition occurs in stages Swallowing, or
           ing the entrance to the nasal cavity when solid or liquid     phate) and organic substances. The submandibular                   of sneezed saliva or by sharing utensils or food contami-              deglutition, occurs as the bolus of macerated, saliva-
           is forced to the back of the throat. The tonsils, at the      glands produce thicker, ropey saliva with similar ion              nated with droplets of saliva. Cases of mumps have                     mixed pizza is moved to the back of the throat. The
           back of the oral cavity, are your first line of defense       content but a larger concentration of mucus. When the              dropped steadily since 1967, when the mumps, measles,                  tongue positions the bolus at the opening to the esoph-
           against any microbes that may enter your mouth along          sympathetic nervous system is active, watery secretion             and rubella (MMR) vaccine was introduced. MMR is                       agus, where you consciously decide to swallow the pizza.
           with the pizza. When bacteria invade the oral cavity, the     from the parotid glands is inhibited, whereas the sticky           now part of routine infant vaccinations.                               This is the last muscular movement you control until
           tonsils swell as they attempt to destroy the pathogen         submandibular secretion is not. This leaves us with the                      Although the mumps virus is uncomfortable in                 the pizza has worked its way to the other end of your GI
           through the action of specific immune tissues.                familiar “cotton mouth” feeling that we associate with             young children, it can be severe in postpubescent indi-                tract. The tongue is composed of voluntary, consciously
                                                                         nervousness.                                                       viduals. The virus usually settles in the parotid salivary             controlled skeletal muscle. The muscularis of the GI
           MALT is a disease-prevention tissue Food is                            In addition to water and ions, saliva contains            glands, causing them to swell and feel jelly-like. In ado-             tract is smooth muscle, controlled by the autonomic
           rarely sterile, and yet we almost never suffer disease from   lysozyme, a bacteriolytic enzyme that helps destroy                lescent males, the testes are often affected, leading to               nervous system. At the very end of the tract, the anal
           ingesting it. Starting with the tonsils, the mucosa of the    bacteria in the oral cavity. Another important compo-              painful swelling but rarely sterility. Mumps may also                  sphincter is again skeletal muscle.
           GI tract contain a disease-prevention tissue called MALT      nent of saliva is salivary amylase, a digestive enzyme             cause swelling or inflammation of the pancreas, brain,                          Swallowing has three stages, two of which are
           (mucosa-associated lymphatic tissue). MALT is also            that breaks carbohydrate poly-                                     meninges, or ovaries. Encephalitis (swelling of the                    shown in Figure 13.10. During the voluntary stage,
           prevalent in the small intestine, large intestine, and ap-    saccharides into monosaccha-        Bacteriolytic                  brain tissue) can be life-threatening and may result in                you consciously swallow the pizza. During the pharyn-
           pendix. These nodules of lymphatic tissue prevent dis-        rides. Amylase occurs in low        Agent that lyses or            permanent damage. Fortunately, this is a rare complica-                geal stage, the bolus involuntarily passes through the
           ease from taking over the lumen of the digestive tract        levels in saliva and in larger      destroys bacteria.             tion of mumps. Hearing loss may also occur in mumps,                   pharynx. During the esophageal stage, the trachea is
           and are important for preserving homeostasis. MALT tis-       quantities in pancreatic secre-                                    but it is often temporary. As we vaccinate more infants,               closed to allow the bolus to pass the larynx and enter
           sues represent a large percentage of the entire immune        tions. As we chew the pizza                                        mumps could become a disease of the past, following                    the esophagus. It is here that the uvula covers the nasal
           system, including about half of the body’s total lympho-      crust, salivary amylase begins breaking the large carbo-           the same pattern as German measles and polio.                          opening and the larynx moves upward against the
           cytes and macrophages. Without MALT, pathogens could          hydrates into the small monosaccharides that cells can
           grow within the digestive tract, penetrate the epithelial     absorb further down the GI tract.
           lining, and cause serious internal infections.                         Mumps, a common disease of the salivar y
                    Although MALT is effective, it can be overrun.       glands, causes swelling of the glands, sore throat, tired-
           Bacteria ingested with food suddenly enter a warm,            ness, and fever (Figure 13.9). Mumps spreads from
           moist, nutrient-rich environment, and they can bloom
           and overwhelm the body’s ability to combat them. Of-
           ten the acid environment of the stomach will kill these                                                                                    Nasopharynx

           blooming bacteria, but sometimes even that is not                                                                                          Hard palate
                                                                                                                                                      Soft palate                                                   Bolus
           enough. If the bacterial colony survives the stomach,                                                                                      Uvula
           the body may flush the entire tract with diarrhea or                                                                                                                                                     Tongue
                                                                                                                                                      Oropharynx
           vomiting to help the specific immune system rid the
                                                                                                                                                      Epiglottis
           body of the invading bacterium.                                                                                                            Laryngopharynx


           The salivary glands aid in digestion The sali-                                                                                             Larynx

           vary glands, located within the oral cavity, secrete wa-
           tery saliva, normally in small quantities to moisten the                                                                                   Esophagus
           oral mucosa. As soon as we smell the pizza, however,
           salivary production increases. Even the thought of food                                                                                                  A   Position of structures before swallowing             B   During the pharyngeal stage of swallowing
           can increase saliva production. When food is in the
           mouth, excess saliva is needed to mix with the food and       Mumps in a young child Figure 13.9                                             Swallowing and the pharynx Figure 13.10
           form the slippery bolus required for swallowing.
                                                                         No longer the threat it was in the 1950s, mumps causes painful               The first two stages of deglutition are seen here. As the bolus of food is swallowed, the larynx moves up, in
                    The major salivar y glands are the parotid
                                                                         swelling of the salivary glands, most often the parotid glands.              turn shifting the position of the epiglottis during the esophageal stage. The bolus of food then slides past the
           glands, located below and in front of the ears, and the
                                                                         In older children and adults, mumps is far more serious, and can             larynx and on to the esophagus. The wave of contraction begun here continues through the entire system,
           submandibular glands under the tongue. The parotid
                                                                         also cause swelling of the brain, pancreas, testes, or ovaries.              pushing this mouthful into the stomach and eventually on to the remaining organs of the GI tract.
           glands produce watery saliva that includes some ions


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           epiglottis. The epiglottis covers the opening to the res-     the stomach). The esophagus terminates at its lower                THE STOMACH PUTS FOOD                                            sphincter refuses to open, the contents of the stomach
           piratory system, and the bolus slides back toward the         end with a sphincter muscle. A sphinter muscle is a cir-           TO THE ACID TEST                                                 are instead ejected through the weaker lower
           esophagus instead of dropping into the respiratory sys-       cular muscle that closes off a tube, functioning like a                                                                             esophageal sphincter, leaving the body at impressive
           tem. Talking while eating can cause the epiglottis to         rubber band pulled tightly around a flexible straw.                The next organ the pizza will encounter in the digestive         speed.
           spasm, because it must be opened to allow air to escape       They appear many times along the GI tract, dividing                system is the stomach, a J-shaped organ that lies be-
           in order to vocalize, but must be closed to prevent the       one organ from the next. The lower esophageal sphinc-              neath the esophagus. The stomach is divided from the             Histologically speaking, the stomach is “the
           bolus from sliding into the respiratory tract. Because        ter (LES) at the base of the esophagus opens as the                esophagus and the small intestine by two sphincter               pits” The typical structure of the gastrointestinal
           the epiglottis cannot be opened and closed at the same        pizza bolus touches it, dropping the bolus into the up-            muscles. The lower esophageal sphincter indicates the            tract undergoes modification at the stomach ( Fig-
           time, it spasms and choking can result, sometimes re-         per portion of the stomach. You can listen to water trav-          upper boundary of the stomach, and the pyloric sphinc-           ure 13.12). The muscularis is usually composed of
           quiring assistance to remove the misplaced bolus.             eling through the esophagus and hitting the LES if you             ter marks the end of the stomach. The pyloric sphinc-            two layers of muscle, one longitudinal and one circu-
                                                                         have a stethoscope. Place the bell of the stethoscope              ter is the strongest sphincter muscle of the digestive           lar. The stomach has a third layer of muscle, called the
                                                                         near your xyphoid process and swallow a mouthful of                tract, opening to allow chyme to enter the small intes-          oblique layer. The function of the stomach is to churn
           THE ESOPHAGUS CONNECTS THE ORAL                               water. You should be able to count to 10, then hear the            tine only when chemically ready. This sphincter is so            and mix the accumulated pieces of pizza mixing the
           CAVITY WITH THE STOMACH                                       water splash against the lower esophageal sphincter. If            powerful that it can cause projectile vomiting in infants.       bolus with the acid environment of the stomach and
                                                                         you are lucky, you might hear the water splash again as                                      The stomach contracts force-           begin digestion of proteins. The oblique layer helps
           The esophagus is a collapsible 20- to 25-centimeter long      it enters the stomach when the LES opens.                                                    fully to push the food into the        this churning and mixing. Because the stomach is a
           conduit that connects the oral cavity with the stomach                 The esophagus runs right through the di-                     Chyme
                                                                                                                                                                      small intestine, but the pyloric       holding area for food as it is ingested, it must be able
           (Figure 13.11). Once the bolus of pizza arrives at            aphragm at the esophageal hiatus. Occasionally a por-                 The thick, partially
                                                                                                                                                                      sphincter remains closed until         to expand. The walls of the stomach contain folds, or
           the top of the esophagus, a peristaltic wave begins. This     tion of the upper stomach can protrude through this                   digested fluid in
                                                                                                                                                                      the chyme is fluid enough to           rugae, that permit expansion somewhat like a deflated
           wave will push the bolus along the esophagus in a con-        opening, resulting in a hiatal hernia. This condition                 the stomach and
                                                                                                                                                                      be passed on. If the pyloric           punching ball.
           trolled manner (neither food nor drink free-fall into         can be painful and often requires medical intervention.               small intestine.



                                                                                                                                                                                                          Esophagus
                                                                         The esophagus Figure 13.11
                                                                       The esophagus is a straight tube, most representative of the four                                                                                                FUNDUS
                                                                       layers of the GI tract. There are no modifications of the tract in                       Lower
                                                                                                                                                                esophageal
                                                                       this organ, which ends with the lower esophageal sphincter.                                                                                                      Serosa
                                                                                                                                                                sphincter
                                                                                                                                                                             CARDIA                                                     Muscularis:

                                                                                                                                                                             BODY                                                         Longitudinal layer

                                                                                                                                                                             Lesser
                                                                                                                                                                             curvature                                                    Circular layer
                                                                                                                                                                PYLORUS
                                                                                                                                                                                                                                          Oblique layer



                                                                                                                                                                                                                                   Greater curvature




                                                                                                                                                                                         Pyloric
                                                                                                                                                          Duodenum                       sphincter               Rugae of mucosa



                                                                                                                                                                                            Anterior view of regions of stomach

                                                                                                                                                            The stomach Figure 13.12


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                   A final modification of the stomach is due to                parietal cells. The chief cells secrete pepsinogen and
           the chemical environment in the organ, where the pH                  gastric lipase. Pepsinogen is an inactive precursor of
                                                                                                                                                 Phases of gastric digestion                              Figure 13.14
           is only 2. Such high acidity breaks down large macro-                the enzyme pepsin, which digests proteins, and there-                                                                                                                            www.wiley.com/




                                                                                                                                                                                                                                                                                         Process Diagram
                                                                                                                                                                                                                                                                 college/ireland
                                                                                                                                                 The activation of the stomach includes three phases.
           molecules and destroys many microbes, but it can also                fore must be secreted in inactive form. (If pepsin itself
           harm the stomach lining. Furthermore, the stomach                    were produced in stomach cells, it would digest the pro-         ● Cephalic phase. In the first phase, thoughts of food
                                                                                                                                                 1
           also secretes enzymes that digest protein, which is what             teins of those cells.) Pepsinogen forms pepsin only un-              and the feel of food in the oral cavity stimulates
           the stomach walls are composed of. Therefore, the                    der pH 2. The parietal cells produce hydrochloric acid               increased secretion from the gastric pits. The
                                                                                                                                                     stomach also begins to churn more actively in
           stomach must be protected from its own contents. The                 and intrinsic factor. The hydrochloric acid is responsi-                                                                        1   Food
                                                                                                                                                     preparation for the incoming food.
           stomach does this by producing a protective layer of                 ble for the acidic pH of the stomach, which both acti-
           thick, viscous, alkaline mucus. Nowhere else does the                vates pepsin and kills microbes. Intrinsic factor is neces-      ● Gastric phase. When the bolus reaches the
                                                                                                                                                 2
           digestive tract need, or produce, such a mucus coating.              sary for the absorption of vitamin B12, a micronutrient              stomach, the second phase of gastric digestion
                                                                                                                                                     begins. Here the stomach produces gastrin as well
                   The walls of the stomach contain gastric pits,               that helps produce blood cells. Although intrinsic fac-
                                                                                                                                                     as continuing the production of pepsin and HCl.                                                     Increased Gastric
           which secrete 2 to 3 quarts of gastric juice each day                tor is produced in the stomach, it is active in the small                                                                                                                    Secretion
                                                                                                                                                     Gastrin aids in stimulation of the gastric pits,
                                   ( Figure 13.13). These pits                  intestine.
                                                                                                                                                     providing a feedback system that speeds digestion.                                       Gastrin
           Gastric                 are composed of chief cells and                       As the pizza is churned in the stomach, gastric             Impulses from the stomach also go back to the
           indicates a                                                          lipase will continue the chemical breakdown of fats that             brain, maintaining contact with the nervous system.
                                                                                began in the mouth. This enzyme specializes in digest-                                                                                                 Food
           relationship to
                                                                                ing short fatty acids such as those found in milk, but           ● Intestinal phase. In the final phase of gastric
                                                                                                                                                 3
                                                                                                                                                                                                                2

           the stomach.                                                                                                                              digestion, the chyme begins to leave through the
                                                                                works at an optimum pH of 5 or 6. In adults, both gas-




                                                                                                                                                                                                                              d
                                                                                                                                                                                                                               Foo
                                                                                                                                                     pyloric sphincter. As the chyme leaves the stomach,                                      Hormones
                                                                                tric lipase and lingual lipase have limited roles.                   gastrin production decreases, the impulses to the
                                                                                         In the stomach, the pizza bolus is converted to             brain indicate a lessening of chyme, and the brain                                                  Decreased Gastric
                                                                                a pasty, liquid chyme. Pepsinogen is converted to                    begins to slow the stimulation of the gastric pits.        3                                            Secretion
                                                                                pepsin and digests the proteins of the tomato sauce and              At the same time, hormones from the beginning
                                                                                the cheese. The low pH assists in denaturing proteins                portion of the small intestine initiate activation
                                                                                and breaking down the remaining macromolecules,                      of the small intestine.
                                                                                providing an easy substrate for digestion in the small
                                                                                intestine.
                                                                                         The stomach is an active organ. As the bolus of      Gastric digestion includes three phases                               motion, causing an increase in both gastric wave force
                                                                                food reaches the stomach, small mixing waves are initi-       Digestion occurs in three phases in the stomach (Fig-                 and secretion from the gastric pits. As chyme is pushed
                                                                                ated. These waves occur ever y 15 seconds or so and           ure 13.14). During the cephalic phase, digestion                      past the pyloric sphincter, stomach volume decreases
                                                                                help to break up the pizza. Even with these mixing            consists of reflexes initiated by the senses, as the name             and stretch receptors begin to relax. This in turn di-
                                                                                waves, the pizza may stay in the fundus of the stomach        implies. This phase started when you ordered the pizza,               minishes the intensity of the gastric phase.
                                                                                for as long as an hour before being moved into the            intensified as you got out the utensils to eat it, and                        The final phase of gastric digestion is the in-
                                                                                body of the stomach. There the pizza mixes with the           peaked as you smelled the pizza after deliver y. The                  testinal phase. As chyme passes through the pyloric
                                                                                gastric secretions and becomes soupy and thin. The            scents and sounds associated with eating stimulate spe-               sphincter, intestinal receptors are stimulated. These re-
                                                                                mixing waves of the stomach become stronger, intensi-         cific portions of the medulla oblongata, which in turn                ceptors inhibit the actions of the stomach, causing it to
                                                                                fying as they reach the pyloric sphincter. With each          trigger secretion of the gastric pits. The parasympa-                 return to rest. At the same time, these receptors stimu-
                                                                                wave, a small portion of the chyme is forced through          thetic nervous system is activated, increasing stomach                late digestion in the small intestine.
                                                                                the pyloric sphincter and into the small intestine. The       movement. Interestingly, these reflexes can be damp-                          Once in the small intestine, the chyme itself
                                                                                rest of the chyme washes back                                 ened by stimulation of the sympathetic nervous system.                stimulates the release of hormones. Chyme containing
                                                                                toward the body of the stom-          Fundus                  Anger, fear, or anxiety opposes the parasympathetic                   glucose and fatty acids, such as the chyme from the
             Gastric pits Figure 13.13                                          ach to be churned further             The portion of          nervous system, shutting down the cephalic phase and                  pizza, causes the release of cholecystokinin (CCK) and
                                                                                with the next mixing wave.            any hollow organ        reducing your feelings of hunger.                                     secretin. CCK inhibits stomach emptying, whereas se-
           Gastric pits are composed of chief cells and parietal cells. These                                                                                                                                       cretin decreases gastric secretions. Both of these also af-
                                                                                                                     lying above the                   Once food enters the stomach, stretch recep-
           cells are responsible for the creation of the specialized
                                                                                                                     opening.                 tors and chemoreceptors are activated, initiating the                 fect the liver, pancreas, and gall bladder, the accessory
           environment of the stomach.
                                                                                                                                              gastric phase. Hormonal and neural pathways are set in                organs of the gastrointestinal tract. The combined


           424     CHAPTER 13 The Digestive System                                                                                                                                                         The Digestive System Processes Food From Start to Finish                425
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           action of these hormones holds the pizza in the stom-                                                                          which creates a longer pathway through the intestine,                brane increase the cell’s surface       Apical
           ach for a prolonged period, ensuring the pizza is suffi-                                                                       allowing more time to absorb nutrients.                              area. The microvilli are small and      membrane
           ciently broken down, despite its high level of hydropho-                                                                                The small intestine has an interesting histology.           difficult to resolve under a light      Membrane at the
           bic fats.                                                                                                                      Because the whole point of the organ is to provide a                 microscope, where they look like        free end, or top, of
                     After 2 to 4 hours, the stomach has emptied,                                                                         surface area for absorption, the small intestine has                 a fuzzy line, not individual struc-     the intestinal cells.
           and all the chyme has entered the small intestine. Be-                                                                         many microscopic projections. The mucosa has finger-                 tures. The entire surface of the
           cause the pizza has a high fat concentration, it will                                                                          like extensions, or villi, each one approximately 0.5 to 1           cell is called a brush border.
           move rather slowly through the stomach, taking closer                                                                          mm long (Figure 13.16). These villi give the inner                   Through an electron microscope, scientists have discov-
           to 4 hours. Had you eaten stir-fried vegetables with their                                                                     surface of the small intestine the look and feel of velvet.          ered even smaller projections on the surface of these
           much lower fat content instead, your stomach would                                                                             Areolar connective tissue is located at the center of                brush borders, which again increase surface area.
           have emptied much more quickly, leaving you feeling                                                                            each villus. This connective tissue supports an arteriole,                    The walls of the small intestine are also dotted
           hungry again after just a few hours.                                                                                           a venule, a blood capillary network connecting the two,              with intestinal glands, which secrete intestinal juice to
                     Sometimes food in the stomach does not                                                                               and a lacteal.                                                       help digestion. The small intestine has an abundance
           “agree” with the stomach because it contains bacteria                                                                                   Beyond the villi, the small intestine also has mi-          of MALT, in the form of Peyers patches, nodules of lym-
           or toxins that irritate the stomach lining. This situation                                                                     crovilli on each apical membrane of the small intesti-               phatic tissue akin to tonsils, embedded in the intestinal
           may cause vomiting. Although not an easy task from a                                                                           nal mucosa. These hairlike projections of the cell mem-              walls (Figure 13.17).
                                                                          Small intestine Figure 13.15
           physiological standpoint, reversing the peristaltic wave
           and churning the stomach violently while holding the         The small intestine is characterized by its velvet-like mucosa.
           pyloric sphincter closed will expel the stomach con-         The entire purpose of this organ is to absorb nutrients,
           tents. The esophageal sphincter is weaker than the py-       requiring a large surface area. The mucosa is thrown into
           loric sphincter and will open first when the stomach         folds, and cells are lined with microvilli and even covered
                                                                        in individual eyelash-like extensions to provide as much
           contents are under pressure. The entire contents of the
                                                                        surface area as possible.
           stomach then return through the esophagus, leaving
           the body via the mouth. The acidity of the stomach is
           not buffered, causing some burning as the fluid passes       shortest of the regions, extending approximately 25 cen-
           the mucus membranes of the mouth and throat. Re-             timeters from the pyloric sphincter. The name duode-
           peated vomiting can be detrimental to the lining of the      num means 12, reflecting the fact that the region is ap-
           mouth as well as the tooth enamel. In addition, replac-      proximately 12 fingers long. The jejunum encompasses
           ing the hydrogen ion concentration in the stomach can        the next meter or so. Jejunum means empty, and this re-
           deplete the hydrogen content of the blood, leading to        gion is characteristically found to be empty during au-
           electrolyte imbalances.                                      topsy. The longest portion, the
                                                                        ileum, is about 2 meters long.
                                                                        The entire length of the small       Mesenteries
           THE SMALL INTESTINE COMPLETES                                intestine is 3 meters, making it     Folds in the lining
           THE NUTRIENT-EXTRACTION PHASE                                the longest digestive organ.         of the abdominal
                                                                        This structure is packed into        cavity that help
           Once in the small intestine, the pizza’s nutrients are fi-   the abdominal cavity by twist-       to secure the                  A villus                                                      Intestinal wall with Peyers patches
           nally ready for absorption. This organ is the only por-      ing and winding around the           digestive organs.              Figure 13.16                                                  Figure 13.17
           tion of the GI tube where nutrients are taken into the       central mesenteries.
                                                                                                                                          Nutrients absorbed by the cells of the intestinal wall are    Peyers patches are an important part of the immune system,
           cells. Prior to reaching the small intestine, the food was
                                                                                                                                          passed through the cell and into the capillary network or     protecting the lumen of the digestive tract from bacterial invasion.
           cut up, broken down, and denatured. Some enzyme ac-          How incredibly large is the surface of the
                                                                                                                                          the lymphatic vessel of the lacteal. Lacteals include         If even one bacterium escaped the stomach, it could potentially cause
           tivity was initiated to break down large macromolecules.     small intestine? Within the small intestine, the                  blind-ended lymphatic capillaries that permit absorption      serious problems here in the nutrient-rich, warm, moist environment
           Here in the small intestine, the nutrients from the pizza    mucosa is shaped into permanent circular folds, which             of ingested fats. Nutrients are usually absorbed directly     of the small instestine. It is the job of these Peyers patches to
           are finally absorbed into the body.                          add important surface area to the organ ( Figure                  into the lacteal capillary system, part of the systemic       prevent these problems from happening.
                    The small intestine has three regions: the duode-   13.15). Not only do these folds increase absorption,              circulatory system.
           num, the jejunum, and the ileum. The duodenum is the         they also force the chyme to move in spiral fashion,


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           Digestion occurs in the small intestine Both                                                                                       ACCESSORY ORGANS HELP                                         cle and liver cells sequestering it. These hormones will
           mechanical and chemical digestion occur in the small                                                                               FINISH THE JOB                                                be covered in Chapter 15, The Endocrine System.
           intestine. Mechanically, the peristaltic wave is modified
           into segmentations and migrating motility complexes.                                                                               Although the gastrointestinal tract provides both a loca-     Ulcers are a hole in the GI tract Ulcers are
           Segmentations are localized mixing contractions that                                                                               tion for nutrient digestion and the surface required to       open wounds that remain aggravated and painful in-
           swirl the chyme in one section of the intestine. They al-                                                                          absorb those nutrients, it cannot complete the job            stead of healing. A gastric or duodenal ulcer is such a
           low the chyme to interact with the walls of the small in-                                                                          alone. Along the length of the tract several accessory        wound in the lining of the GI tract (Figure 13.19).
           testine but do not move it along the tract. Migrating                                                                              organs assist in digestion, including the pancreas, liver,    Gastric ulcers occur in the stomach, whereas duodenal
           motility complexes move the chyme along the length of                                                                              and gall bladder.                                             ulcers are located in the duodenum of the small
           the small intestine. These movements strengthen as the                                                                                                                                           intestine.
           nutrient level in the chyme decreases.                          Microvilli in the small intestine cells                            The pancreas is an enzyme factory The pan-                              The mucous lining that normally protects the
                                            When soupy chyme en-           Figure 13.18                                                       creas functions as an exocrine gland in the digestive sys-    stomach from digestion must be compromised for an
                                   ters the duodenum, digestion of                                                                            tem, producing enzymes that are released via the pan-         ulcer to develop. This can happen when alcohol or as-
           Pancreatic juice        proteins, lipids, and carbohy-        The cells of the small intestine are the only nutrient-absorbing
                                                                                                                                                                                                            pirin enters the stomach because these compounds can
                                                                                                                                              creatic duct. Almost all of the enzymes that act in the
           The fluid produced                                            structures in the digestive system. The larger their surface area,
                                   drates has just begun. Pancre-                                                                             small intestine are made in the pancreas. Pancreatic          degrade the mucous lining. Aspirin labels direct you to
           by the pancreas and                                           the greater the chances that nutrients taken in with the original
                                   atic juice is added to the chyme                                                                           juice also buffers the acidity of the chyme as it leaves      take them with a full glass of water so that the pill is
           released into the                                             food will be absorbed before they pass through the small
                                   as it enters the small intestine,                                                                          the stomach. The small intestine does not have the pro-       washed through the stomach, or dissolved rather than
           small intestine.                                              intestine. The incredibly extensive surface area of these cells
                                   adding a suite of digestive en-       allows fats and nutrients to diffuse or be actively absorbed at a    tective layer of mucus found in the stomach, so it has        left sitting on the mucous layer. If the mucous layer is
                                   zymes that are specific for differ-   high rate.                                                           no protection from the corrosive pH 2 solution being          worn away, the pH of the lumen begins to burn the
           ent macromolecules. Sucrase, lactase, maltase, and pan-                                                                            released from the pyloric sphincter. The pancreas se-         stomach lining, and pepsin will digest proteins of the
           creatic amylase all digest carbohydrates.                                                                                          cretes pancreatic juice into chyme immediately as it en-      stomach cells, creating an ulcer. Although in the past
                     The pH buffers of the pancreatic juice immedi-      transported in the lymphatic capillar y of the lacteal.              ters the duodenum, largely neutralizing the chyme to          ulcers were commonly blamed on stress that caused the
           ately bring the pH of the chyme from 2 back to 7 in the       From here, the fats flow with lymph to the subclavian                safeguard the duodenum from acid burns.                       release of excess stomach acid, many gastric ulcers are
           small intestine to prevent damage to the lining of the        vein. Once in the bloodstream, lipoprotein lipase                             In addition to secreting digestive enzymes into      actually caused by infection with Helicobacter pylori, a spi-
           duodenum. Bringing the pH up to 7 protects the walls          breaks chylomicrons down to short-chain fatty acids and              the digestive tract, the pancreas is also responsible for     ral bacterium that thrives in the highly acidic stomach.
           of the small intestine, but renders pepsin inactive. Pro-     glycerol.                                                            secreting hormones into the bloodstream. The pan-             People who are susceptible to this bacterium often de-
           tein digestion continues using trypsin, chymotrypsin,                  We have probably all heard of LDL (low-density              creas makes insulin and glucagon, which are responsi-         velop gastric ulcers due to bacterial colonies that live
           carboxypeptidase, and elastase, all secreted from the         lipoprotein) and HDL (high-density lipoprotein) cho-                 ble for regulating glucose uptake by the cells. Insulin       on the mucus. Rather than counsel these patients to re-
           pancreas. Protein digestion is completed on the ex-           lesterol, but we may not be aware of their functions.                stimulates glucose uptake, whereas glucagon causes glu-       duce their stress level, the accepted ulcer treatment of
           posed edges of the intestinal cells themselves, using the     Transporting insoluble fats through the aqueous blood-               cose to be released into the bloodstream by those mus-        old, they are given antibiotics to cure their ulcers.
           enzymes aminopeptidase and dipeptidase.                       stream requires a protein carrier. Initially this carrier is
                     In adults, most lipid digestion occurs in the       LDL, which is essentially a small protein carr ying a
           small intestine because lingual lipase and gastric lipase     large fat droplet, hence the term low density. LDL is of-
                                                                                                                                                                                                           Gastric ulcer Figure 13.19
           are barely effective in adults. Pancreatic lipase is the      ten called “bad” cholesterol because higher levels of
           main force causing the breakdown of fats in adults, re-       LDL in your blood indicate a greater proportion of
           moving two of the three fatty acids from ingested             large fat droplets being carted from the lacteals to the
           triglycerides.                                                liver for degradation. LDL can often “drop its load”
                     In the cells of the small intestine, carbohy-       along the way, leading to plaque formation and athero-
           drates, short-chain fatty acids, and amino acids are ab-      sclerosis.
                                   sorbed from the chyme and                      HDL, on the other hand, is sometimes called
                                   transported to the capillaries of     “good” cholesterol. High-density lipoprotein uses a
           Chylomicrons
                                   the lacteal (Figure 13.18).           large protein to carry a small fat. HDL carries small fats
           Small lipoproteins
                                   Absorbed triglycerides are too        from storage to the muscles and liver, where they are
           carrying ingested
                                   large to pass directly into the       metabolized. Because of the opposing roles of these
           fat from the
                                   bloodstream. They are con-            two carrier molecules, the LDL-to-HDL ratio helps as-
           intestinal mucosa
                                   verted to chylomicrons and            sess cholesterol levels and heart disease risk.
           to the liver.

                                                                                                                                                                                                                                                                    429
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           The liver detoxifies what we add to the                                                                                               in the feces. Hepatitis A also can be transmitted directly      comes increasingly tender, and        Gangrene
           bloodstream The liver is the largest organ aside                                                                                      through kissing or sexual contact. The virus can live for       simple movements cause pain.          Tissue death due to
           from the skin and usually weighs about 1,450 grams.                                                                                   3 to 4 hours on hard surfaces, such as eating utensils.         These are all symptoms of a           lack of blood flow.
           The liver has two lobes, and sits mostly on the right side                                                                            Symptoms of infection include fatigue, nausea, vomit-           blockage in the appendix that
                                     of the body. Within the lobes of                                                                            ing, liver pain, dark urine, and light colored stools. Re-      prevents normal flow through
                                     the liver, the hepatocytes are                                                                              covery usually takes approximately 6 months, during             the large intestine. Feces may be blocking the entrance,
           Hepatocytes
                                     arranged in lobules ( Figure                                                                                which time it is difficult to work or carry out daily duties.   or lymph nodes in the surrounding walls may be
           Liver cells
                                     13.20), designed to allow                                                                                            Hepatitis B is passed from person to person in         swollen due to infection. In either instance, the con-
           (hepato liver;
                                     maximum contact between he-                                                                                 body fluids. This form of hepatitis is serious because it       tents of the appendix cannot move, leading to a
           cyte cell)
                                     patocytes and venous blood. The        Liver lobule Figure 13.20                                            has few symptoms, so individuals can unknowingly                buildup of pressure, decreased blood flow, and inflam-
                                     lobules monitor blood collected                                                                             transmit the virus. After some years, hepatitis B causes        mation. If the pressure is not relieved quickly, the en-
                                                                          Each lobule is composed of a triad consisting of a hepatic portal      permanent liver damage, liver failure, or liver cancer.         tire organ can rupture or suffer gangrene. For un-
           from the small intestine, adding and subtracting mate-
                                                                          vein, a hepatic artery, and a bile duct. These structures are
           rials to maintain fluid homeostasis.                                                                                                           Hepatitis C is the most common chronic blood-          known reasons, most cases of appendicitis occur in
                                                                          found in the center of the lobule, with small channels that
                     The liver is served by a portal system. The veins                                                                           borne virus in the United States. It is transmitted             people aged 10 to 30. As soon as inflammation is diag-
                                                                          radiate from the central area to the individual cells of the lobule,
           of the small intestine drain into the liver, where they                                                                               through blood-to-blood contact, like HIV. After suffer-         nosed, the appendix is surgically removed to prevent it
                                                                          similar to the spokes on a bicycle wheel. Fluid within the lobules
           break into capillaries again before being collected into       is cleansed by the hepatocytes and sent on to the vena cavae.
                                                                                                                                                 ing through a flu-like illness just after infection, people     from rupturing and releasing pathogens into the intes-
           a larger vein and returned to the heart. Blood flow                                                                                   infected with hepatitis C are almost completely symp-           tine or the abdominal cavity.
           through the digestive organs travels from arteries to                                                                                 tom-free. After many years, liver damage begins to show                  The remainder of the large intestine is
           capillaries to veins on to the liver, where it moves back      cium ions from the concentrated bile, resulting in the                 up, and the damage progresses. It is most accurately di-        commonly called the colon. The four divisions of the
           to capillaries before going on to the veins that return to     formation of a stone. Stones can grow big enough to                    agnosed via tests for antibodies. Although there is no          colon describe the direction of flow within them. The
           the heart. This portal system gives the hepatocytes ac-        get stuck in the bile duct when the gall bladder releases              cure, treatment includes maintaining a healthy diet and
           cess to the fluid composition of the blood coming from         its contents. This causes pain and blocks the flow of                  exercise program. Hepatitis C is a major cause of liver
           the small intestine. This blood includes all absorbed          bile. The gall bladder is often removed if stones are a                transplants.
                                                                                                                                                                                                                   Large intestine Figure 13.21
           compounds, nutrients, as well as toxins, from the small        chronic problem. After removal, bile can be produced
           intestine. The hepatocytes must cleanse the blood be-          but not stored. The patient should not eat fatty meals,                Chyme passes into the large intestine Once                      The four parts of the colon can be easily seen in this image.
           fore it reaches the heart, removing toxins and storing         because there is no store of bile to aid in lipid digestion.           the pizza that we ate hours ago reaches the end of the          A radiopaque substance was given to the patient that reflects X
           excess nutrients, such as iron, and fat-soluble vitamins                                                                              small intestine, the body cannot pull any more nutri-           rays. Exposure to X rays then provides a clear view of the colon
           such as A, D, and E.                                           Liver diseases can be deadly Because the liver                         ents from it. The chyme now passes from the small in-           itself. The ascending colon in on the left in the image, the large
                     Cholesterol, plasma proteins, and blood lipids       serves as a detoxification center for blood coming from                testine into the next portion of the GI tract, the large        downward loop is the transverse colon, and the very densely
                                                                                                                                                                                                                 staining descending colon runs along the right side of the
           are manufactured in the hepatocytes. The liver also            the intestinal tract, it is exposed to many toxic sub-                 intestine (Figure 13.21). The overall function of
                                                                                                                                                                                                                 image. The sigmoid colon makes its characteristic “S” turn
           monitors the glucose level in the blood; when it ex-           stances, and the liver can be damaged by the very sub-                 the large intestine is to reabsorb the water that was
                                                                                                                                                                                                                 at bottom center of the image.
           ceeds 0.1%, hepatocytes remove and store the excess as         stances it is detoxifying. This occurs in the disease cir-             added to the chyme to begin digestion. Along with the
           glycogen. When the glucose level drops, stored glyco-          rhosis of the liver. Cirrhosis is a general term for a series          water, the large intestine absorbs many dissolved miner-
           gen is broken down and released from the hepatocytes,          of events that cause scar tissue to build up in the liver.             als and some vitamins. The valve that makes the transi-
           and glucose again rises in the blood.                          The scar tissue impedes the blood flow through the lob-                tion from the ileum of the small intestine to the cecum
                     Bile is formed by the liver as a by-product of the   ules, preventing hepatocytes from doing their job.                     of the large intestine is called the ileocecal valve. The
           breakdown of hemoglobin and cholesterol. It is stored          Detoxification of intestinal blood is impaired. Cirrhosis              ileum joins the large intestine a few centimeters from
           in the gall bladder, under the right lobe of the liver.        can be caused by alcohol consumption, chronic hepati-                  the bottom. The cecum hangs below the junction,
           Bile salts from the gall bladder are released when fatty       tis infection, autoimmune diseases that attack the liver,              forming a blind pouch that ends in the vermiform
           chyme is present in the duodenum, such as that from            or even congenital defects.                                            appendix.
           the greasy cheese pizza. The concentrated bile salts act                Other common liver diseases are viral, includ-                         Although the function of the appendix is un-
           as an emulsifier or biological detergent, breaking             ing hepatitis A, B, and C.                                             clear, it may play a role in the immune system. When
           larger fat globules into smaller ones. Bile aids in fat di-             Hepatitis A is transmitted through drinking wa-               the appendix acts up, we get appendicitis, which pre-
           gestion by increasing the surface area on which the di-        ter or eating foods contaminated with infected fecal                   sents as pain near the belly button that migrates to the
           gestive activities of pancreatic lipase can act.               matter. Unlike other hepatitis viruses, hepatitis A virus              lower right side. Other symptoms include nausea, vom-
                     Stones can form in bile. A small crystal of cho-     remains intact as it passes through the stomach and the                iting, low fever, constipation or diarrhea, inability to
           lesterol that forms in the gall bladder may attract cal-       intestinal tract. It is still virulent after leaving the body          pass gas, and abdominal bloating. The abdomen be-

           430    CHAPTER 13 The Digestive System
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                                                                                                                                                                                                                                                                                      Obligate
             Polyps                                                                                                                                                                                                                                                                   anaerobes
                                      ascending colon runs up the                         vent mass movements are often helpful in slowing the                        the colon, water is reabsorbed from the soupy chyme,                      the colon because they break
             Growths protruding                                                                                                                                                                                                                                                    Bacteria that
                                      right side of the abdominal                         movement of chyme through the large intestine, giving                       concentrating the waste material and conserving fluid.                    down indigestible material and
             from a mucous                                                                                                                                                                                                                                                         require an oxygen-
                                      cavity. The transverse colon                        the walls of the organ ample time to return the excess                      As the water is pulled back into the bloodstream across                   often produce essential vita-
             membrane.                                                                                                                                                                                                                                                             free environment.
                                      cuts across the top of the ab-                      water to the bloodstream. In severe diarrhea, remedies                      the lining of the colon, so too are minerals and vita-                    mins. Sometimes these colonies
                                      dominal cavity, posterior to                        that contain minerals and fluid are ingested to replace                     mins. The removal of water leaves undigested remains                      can be embarrassing because
           the stomach. At the left side of the abdominal cavity the                      what is lost in the diarrhea.                                               of food and fiber in the colon, as well bacteria, such as                 they generate gas when fermenting solids.
           colon turns back down, in the descending colon. At the                                  The last 20 centimeters of the colon are the                       E. coli and other obligate anaerobes that naturally live                          See Table 13.5 for a summary of the organs
           lower left of the abdominal cavity, the colon makes an S                       rectum and anus. Chyme remains in the colon for 3–10                        in the large intestine. These colonies are necessary in                   involved in digestion.
           turn to wind up in the center of the body. This turn is                        hours, during which time it progressively becomes
           called the sigmoid colon and is the portion of the colon                       drier. Compacted chyme is referred to as feces. When
           where feces often sit for long periods of time before                          feces enter the upper portion of the rectum, they trig-                       CONCEPT CHECK
           moving out the rectum. Often, polyps can develop in                            ger the opening of the internal anal sphincter, a
           the colon as feces sit against the mucosa.                                     smooth muscle. The feces move into the rectum and
                                                                                                                                                                          List the four parts of the              Detail the function of        What is the function of
                   The walls of the large intestine have haustra,                         press against the external anal sphincter. This triggers                            colon.                                  the gastric pits. List       the pancreas?
           pouches created by strands of muscle in the walls.                             defecation, a skeletal muscle action. As with all skeletal                                                                  the cells of these pits
           These pouches fill with undigested material, which                             muscles, control over defecation is voluntary. On aver-                                                                     and their secretions.
                                                                                                                                                                                                                                                List the four parts to the
           moves from pouch to pouch via mass movements.                                  age, by age 2 1/ children are mature enough to control
                                                                                                           2                                                                                                                                       large intestine.
                   Diarrhea results from an irritation of the colon.                      defecation.
           The chyme moves through the colon far too quickly for                                   Material moves through the large intestine in
           water or minerals to be absorbed. Medicines that pre-                          mass movements, created using a peristaltic wave. In


              Summary of the digestive organs Table 13.5
              Organ
              Mouth
                                        Functions
                                        See other listings in this table for the functions of the tongue, salivary glands, and teeth, all of which are in the
                                                                                                                                                                      Digestion Is Both Mechanical and Chemical
                                        mouth. Additionally, the lips and cheeks keep food between the teeth during mastication, and buccal glands lining
                                        the mouth produce saliva.                                                                                                                                                                               occurred. They have all the properties and chemical
                                                                                                                                                                        LEARNING OBJECTIVES
              Tongue                    Maneuvers food for mastication, shapes food into a bolus, maneuvers food for deglutition, detects taste and touch                                                                                       bonds of the original apple, but with a larger surface
                                        sensations, and initiates digestion of triglycerides.                                                                           Define mechanical and chemical digestion.
                                                                                                                                                                                                                                                area needed for chemical digestion.
              Salivary glands           Produce saliva, which softens, moistens, and dissolves foods; cleanses mouth and teeth; and initiates the digestion of          List the major enzymes of chemical digestion, and note their                     Mechanical digestion occurs mainly in the
                                        starch.                                                                                                                         substrate.
                                                                                                                                                                                                                                                mouth. Once the bolus of food is passed to the esopha-
              Teeth                     Cut, tear, and pulverize food to reduce solids to smaller particles for swallowing.
                                                                                                                                                                                                                                                gus, a small amount of mechanical digestion occurs in
              Pharynx                   Receives a bolus from the oral cavity and passes it into the esophagus.
                                                                                                                                                                                                                                                the stomach, as it rolls and churns the food into chyme.
              Esophagus                 Receives a bolus from the pharynx and moves it into the stomach. This requires relaxation of the upper esophageal                             hroughout this look at the digestive sys-                 The chyme then moves through the pyloric sphincter


                                                                                                                                                                        T
                                        sphincter and secretion of mucus.
                                                                                                                                                                                      tem, we have discussed various organs and                 into the duodenum, where large droplets of fat are
              Stomach                   Mixing waves macerate food, mix it with secretions of gastric glands (gastric juice), and reduce food to chyme. Gas-
                                        tric juice activates pepsin and kills many microbes in food. intrinsic factor aids absorption of vitamin B12 . The stom-                      their contribution to the process of diges-               emulsified via bile. The action of bile is a form of me-
                                        ach serves as a reservoir for food before releasing it into the small intestine.                                                              tion. Now it’s time to summarize, so that                 chanical digestion, breaking larger fat droplets into
              Pancreas                  Pancreatic juice buffers acidic gastric juice in chyme (creating the proper pH for digestion in the small intestine),         we can view digestion as one continuous process.                          smaller ones without altering the chemical structure of
                                        stops the action of pepsin from the stomach, and contains enzymes that digest carbohydrates, proteins, triglycerides,
                                        and nucleic acids.
                                                                                                                                                                               Digestion is the breaking down of food into                      the fats. At this point, the chyme is ready for enzymatic
                                                                                                                                                                      substances that can be absorbed and used by the body.                     degradation, and mechanical digestion is finished.
              Liver                     Produces bile, which is needed for the emulsification and absorption of lipids in the small intestine.
                                                                                                                                                                      This is accomplished through two processes: mechani-                               Unlike mechanical digestion, enzymatic diges-
              Gallbladder               Stores and concentrates bile and releases it into the small intestine.
                                                                                                                                                                      cal digestion and chemical (or enzymatic) digestion.                      tion alters chemical bonds. Most of the food we ingest
              Small intestine           Segmentations mix chyme with digestive juices; migrating motility complexes propel chyme toward the ileocecal
                                        sphincter; digestive secretions from the small intestine, pancreas, and liver complete the digestion of carbohydrates,
                                                                                                                                                                      Mechanical digestion refers to the chopping, cutting,                     is composed of polymers, long chains of repeating sub-
                                        proteins, lipids, and nucleic acids; circular folds, villi, and microvilli increase surface area for absorption; site where   and tearing of large pieces of food into smaller ones.                    units, which our digestive enzymes must break into
                                        about 90% of nutrients and water are absorbed.                                                                                                                                                          short chains or monomers. It is these shorter units that
                                                                                                                                                                      Bites of apple, for example, are crushed and torn into
              Large intestine           Haustral churning, peristalsis, and mass peristalsis drive the contents of the colon into the rectum; bacteria produce        pieces in your mouth, but these pieces are still recog-                   are absorbed in the small intestine and used to produce
                                        some B vitamins and vitamin K; absorption of some water, ions, and vitamins; defecation.
                                                                                                                                                                      nizable as apple pieces, and no chemical alteration has                   the proteins and energy needed for survival.


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                        Hydrolase activity                  Figure 13.22                                                                                        Digestive enzymes Table 13.6
                                                                                                                                                                Enzyme                         Source                          Substrates                            Products
   Process Diagram
                        Most of our digestive enzymes are
                                                                                                                                                                SALIVA
                        hydrolases, meaning that they                      Smaller                                                     Large
                                                                           molecules                                                   molecule                 Salivary amylase               Salivary glands.                Starches (polysaccharides).           Maltose (disaccharide), mal-
                        catalyze the breakdown of large                                                                                                                                                                                                              totriose (trisaccharide), and
                        polymers by inserting water                                                                                                                                                                                                                    –dextrins.
                        molecules between monomers. We                                                                                                          Lingual lipase                 Lingual glands in the tongue.   Triglycerides (fats and oils) and     Fatty acids and diglycerides.
                        unconsciously know that digesting                                                                                                                                                                      other lipids.

                        requires water because we find it                                             Enzyme                                                    GASTRIC JUICE
                        uncomfortable to eat without                                                                                   1                        Pepsin (activated from         Stomach chief cells.            Proteins.                             Peptides.
                                                                                                                                                                pepsinogen by pepsin and
                        drinking.                                                                                                                               hydrochloric acid)

                        ● Large molecule of food enters the
                        1                                                                                                                                       Gastric lipase                 Stomach chief cells.            Triglycerides (fats and oils).        Fatty acids and monoglycerides.
                           digestive system.                                                                                                                    PANCREATIC JUICE




                                                                           Enzyme
                        ● Enzyme binds to food (substrate)




                                                                                                                              Enzyme
                        2                                                                                                                                       Pancreatic amylase             Pancreatic acinar cells.        Starches (polysaccharides).           Maltose (disaccharide),
                                                                                    3
                                                                                                                        2                                                                                                                                            maltotriose (trisaccharide), and
                           molecule.
                                                                                                                                                                                                                                                                      –dextrins.
                        ● Enzyme uses H O to split the
                        3                 2                                                                                                                     Trypsin (activated from        Pancreatic acinar cells.        Proteins.                             Peptides.
                           substrate molecule in half, leaving                                                                                                  trypsinogen by enterokinase)
                           an OH on one product molecule                                                                                                        Chymotrypsin (activated from   Pancreatic acinar cells.        Proteins.                             Peptides.
                           and an H on the other.                                                                                                               chymotrypsinogen by trypsin)
                                                                                                                                                                Elastase (activated from       Pancreatic acinar cells.        Proteins.                             Peptides.
                                                                                                                                                                proelastase by trypsin)
                                                                                                                                                                Carboxypeptidase (activated    Pancreatic acinar cells.        Amino acid at carboxyl end of         Amino acids and peptides.
                                                                                                                                                                from procarboxypeptidase                                       peptides.
                              In order to digest our myriad foodstuffs, we              for nutrients. When it literally                Amoeba                  by trypsin)
                     need several digestive enzymes (Figure 13.22). As                  runs across a bit of organic mate-              A single-celled         Pancreatic lipase              Pancreatic acinar cells.        Triglycerides (fats and oils) that    Fatty acids and monoglycerides.
                                                                                                                                                                                                                               have been emulsified by bile salts.
                     you know, enzymes are functional proteins that work                rial, the amoeba engulfs the parti-             organism that
                     best under a set of optimal conditions of pH, tempera-             cle and brings it into its body via             moves using             Nucleases
                                                                                                                                                                 Ribonuclease                  Pancreatic acinar cells.        Ribonucleic acid.                     Nucleotides.
                     ture, substrate and product levels. (The substrate is the          phagocytosis. Once inside the                   pseudopods (false
                                                                                                                                                                Deoxyribonuclease              Pancreatic acinar cells.        Deoxyribonucleic acid.                Nucleotides.
                     compound the enzyme acts upon, and the product is                  amoeba, the particle is broken                  feet formed by
                                                                                                                                                                BRUSH BORDER
                     the result of that enzymatic action.) All enzymes are              into its building blocks by diges-              oozing a portion of
                                                                                                                                                                 –Dextrinase                   Small intestine.                 –Dextrins.                           Glucose.
                     specific for a particular substrate and catalyze only one          tive enzymes in the lysosome                    the body forward).
                                                                                                                                                                Maltase                        Small intestine.                Maltose.                              Glucose.
                     reaction.                                                          (Figure 13.23). Monosaccha-
                              Enzyme names are usually built from the name                                                                                      Sucrase                        Small intestine.                Sucrose.                              Glucose and fructose.

                     of the substrate, followed by the suffix “-ase”. For exam-                                                                                 Lactase                        Small intestine.                Lactose.                              Glucose and galactose.

                     ple, lipase digests lipids, and nucleases digest nucleic                                                                                   Enterokinase                   Small intestine.                Trypsinogen.                          Trypsin.
                     acids. The major digestive enzymes, along with their                                                                                       Peptidases
                     substrate, products, and sources, are listed in Table                                                                                        Aminopeptidase               Small intestine.                Amino acid at amino end of            Amino acids and peptides.
                                                                                                                                                                                                                               peptides.
                     13.6.
                                                                                                                                                                Dipeptidase                    Small intestine.                Dipeptides.                           Amino acids.
                              All digestive enzymes with the exception of two
                                                                                                                                                                Nucleosidases and
                     act in the small intestine. Salivary amylase begins to di-                                                                                  phosphatases                  Small intestine.                Nucleotides.                          Nitrogenous bases, pentoses, and
                     gest carbohydrates in the mouth and continues it in the                                                                                                                                                                                         phosphates.
                     bolus of food entering the stomach. Pepsin, in the
                     stomach, works best at pH 2. The rest of the digestive
                     enzymes operate best at pH 7, and are found inside the                                                                                   rides are released from carbohydrates; amino acids are              ents are obtained by the amoeba in a similar fashion, via
                     small intestine.                                                                                                                         released from proteins; and small carbon compounds                  pinocytosis. Often micronutrients are released from
                              For some organisms, locating and ingesting nu-                                                                                  are released from fatty acids. These small organic com-             larger compounds during lysosomal digestion.
                     trients is relatively simple. The single-celled amoeba                                                                                   pounds are then used by the amoeba to generate essen-                       The human body is far more complex than the
                                                                                         Amoeba eating Figure 13.23
                     oozes through the environment, constantly searching                                                                                      tial enzymes, cellular structures, and energy. Micronutri-          amoeba, but each cell still needs nutrients in order to


                     434    CHAPTER 13 The Digestive System                                                                                                                                                                                Digestion Is Both Mechanical and Chemical                    435
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           survive. Interestingly, human cells absorb nutrients in             proximately 70 to 110 mg glucose per 100 ml blood.
           exactly the same manner as the amoeba: through diffu-               This level is essential to keep neurons functioning.
                                                                                                                                            What is an ideal weight? How far should you go to look skinny?




                                                                                                                                                                                                                                                                                        Ethics and Issues
           sion, osmosis, facilitated diffusion, and active transport          When blood glucose drops, we feel hungry. If we eat,
           (including both phagocytosis and receptor-mediated                  blood sugar levels rise from the absorption of ingested          BMI > 30, or ~ 30 lbs. overweight for 5'4" person                    •     Binge eating disorder also features bouts of uncontrolled
           endocytosis). However, the cells cannot leave their posi-           glucose. If we choose not to eat, we begin to break                                                                                         eating, without the purging phase. For this reason, binge
                                                                                                                                                                      2005
           tions in the tissues to ooze through the environment in             down glycogen stores, where excess glucose has been                                                                                         eating disorder can cause obesity.
           search of nutrients. Although that would make a won-                stored in liver and skeletal muscles. Glycogen can break                                                                                   Eating disorders are not merely “lifestyle diseases.” Ac-
           derful B-movie plot, our cells must remain organized                down to glucose relatively quickly. Fats and proteins can                                                                             cording to federal statistics from 1994, an estimated 10 per-
           and in position! Therefore, the digestive system’s job is           also be converted to glucose, but at a higher energy ex-                                                                              cent of anorexia patients will die of complications, an ex-
           to prepare nutrients for circulation through the blood,             pense. During starvation, the protein of skeletal mus-                                                                                tremely high rate for a psychiatric illness. Although the
           which reaches every cell.                                           cle, and even heart muscle, is broken down to provide                                                                                 exact causes of eating disorders are uncertain, it’s common
                    Regulation of our digestive activities is based on         glucose for the brain, as described in the material on                                                                                to see a history of depression, anxiety, or substance abuse.
           blood sugar levels. Normally, blood sugar is kept at ap-            the general adaptation syndrome in Chapter 9.                                                                                         Eating disorders are also more common among people like
                                                                                                                                                                                                                     models or actors, who need to be thin for occupational rea-
                                                                                                                                                                                                                     sons. The ideal therapy is likewise uncertain and can range
                              CONCEPT CHECK                                                                                                                                                                          from patient education to in-patient treatment in psychiatric
                                                                                                                                                                                                                     hospitals. Treatment seems to work better if begun early.
                                                                                                                                                15%-19%       20%-24%        25%-29%      30%-34%
                                 Where does mechanical               What are the                                                                                                                                         Eating disorders are largely a hidden epidemic, but two
                                     digestion occur?                  products of                                                                                                                                   social factors might make them less common: a reduction in
                                                                       the major                                                            Need to lose weight? In the United States, the rate of obe-              the overall level of overweight and obese individuals, and an
                                                                       pancreatic                                                                                                                                    emphasis on finding a healthy weight, instead of following
                                 List the enzymes that digest          enzymes?
                                                                                                                                            sity is soaring; 30 percent of U.S. residents at least 20 years
                                     protein and specify where                                                                              of age are considered obese. Since 1980, the rate of obesity             the thin-is-beautiful approach of fashion magazines and
                                     they are active.                                                                                       has tripled among young people. But even people with nor-                many movie actresses.
                                                                                                                                            mal weight seem to feel they would be smarter, sexier, and
                                                                                                                                            more lovable if they could dump a few pounds.
                                                                                                                                                  Certainly, being overweight and especially obese is as-
                                                                                                                                            sociated with high rates of hypertension, some cancers, and
                                                                                                                                            type 2 diabetes. But an obsession with being overweight
                                                                                                                                            can take its own toll:Often those who suffer from this are
                                                                                                                                            struck with a general feeling that they are not okay in their
           Nutritional Health and Eating Disorders:                                                                                         appearance or performance, and more specifically, they de-
                                                                                                                                            velop eating disorders.
           You Truly are What You Eat                                                                                                             Eating disorders primarily affect young women; be-
                                                                                                                                            tween 1 and 4 percent of women aged 14–25 suffer from
                                                                                                                                            one eating disorder or another, according to federal figures.
              LEARNING OBJECTIVES                                              fill our digestive system with foods high in necessar y      The most common disorders are:
              Understand how to make healthy eating choices.                   nutrients, our bodies will function at peak levels. Of
                                                                                                                                            •    Anorexia nervosa, a form of self-starvation. Thin people
                                                                               course, we can get too much of a good thing. If we in-            think they are fat and use severe diets, intense exercise,
              List the common eating disorders and their symptoms.
                                                                               gest more calories than we “spend,” regardless of their           or purging in an attempt to lose weight. The physical
                                                                               quality, we will store the excess in adipose tissue as fat        symptoms resemble starvation: osteoporosis, brittle hair,
                                                                                                                                                 intolerance of cold, and muscle wasting, among many
                                                                               (triglycerides).                                                  other possible problems.
                          iet and nutrition are important aspects                       Much attention is given to our diets, and its ef-


             D
                                                                                                                                            •    Bulimia nervosa, secretive eating binges followed by
                          of overall health because most of the                fect on our body, both in the media and in society. As a          vomiting or enemas to clear the food before it can be
                          compounds that enter the body enter                  society, we are obsessed with being thin. For some, this          digested. Because bulimics may not be severely thin,
                          through the digestive system. If we put              obsession takes an unhealthy turn, in the form of two             the disorder can remain undiagnosed for a long while.
                                                                                                                                                 Complications include electrolyte imbalance and acid         www.wiley.com/
           nothing useful into the digestive system, our bodies will           common eating disorders, anorexia nervosa and bu-                 damage to the upper GI tract.                                college/ireland

           not have a good source of raw material for the proteins,            limia nervosa. Both of them stem from the desire to be
           enzymes, and energy required for life. Conversely, if we            thin and therefore “beautiful,” and are described in the


           436     CHAPTER 13 The Digestive System                                                                                                                                           Nutritional Health and Eating Disorders: You Truly are What You Eat                  437
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                                                                                                                                                             Ethics and Issues box. Another problem related to eat-        cause of bacterial diarrhea in the world. Salmonella,
                             Are E. coli Bacteria Hazardous to My Health?                                                                                    ing is even more common: obesity. All of these eating         found in the intestines of birds, reptiles, and mammals,
                                                                                                                                                             disorders grow out of a culture that is obsessed with         causes the usual food poisoning symptoms, but can be-
   I WONDER . . .

                           The healthy human colon is a sea of bacteria, including vast            E. coli O157:H7 normally lives in the intestines of       beauty, and will be hard to resolve without changing          come much more serious if untreated. Salmonella can
                           numbers of Escherichia coli (E. coli). A large percentage of       healthy cattle, and some other ruminants. Most human in-
                           the mass of feces consists of billions of E. coli cells. Inside    fections come from meat contaminated by the contents of        the social view of beauty.                                    escape the intestinal tract and enter the bloodstream,
                           the GI tract, almost all E. coli are helpful or at worst harm-     cattle intestines at the slaughterhouse. Ground beef is the                                                                  leading to septicemia, a life-threatening condition in
                           less. However, E. coli can infect the urinary tract (causing       most common carrier because the bacteria can reside deep                                                                     which the blood carries a poison throughout the body.
                           UTI, a urinary tract infection). If it escapes the colon and en-   inside the meat, where it cannot be washed off or easily       FOOD IS LIFE-SUSTAINING,                                      E. coli is normally present in the colon of cattle, pigs,
                           ters the abdomen, it can cause peritonitis, a serious ab-          heat-sterilized by cooking. More rarely, E. coli O157:H7 can
                           dominal infection. We often hear about outbreaks of E. coli        be spread by an infected person, or in unpasteurized juice
                                                                                                                                                             BUT SOMETIMES IT CAN BE                                       and humans. A toxic form of E. coli is described in the I
                           infections in the media; outbreaks that cause serious illness      or water. The number of bacteria needed to start an E. coli    LIFE-THREATENING AS WELL                                      Wonder box.
                           for a few unfortunate victims, or outbreaks that sweep en-         O157:H7 infection is unknown but seems to be much lower                                                                                The most common viral food contaminant is
                           tire small towns. Recently 146 US citizens in 23 states suf-       than for typical food-borne pathogens.                         Eating disorders are not the only pathologies of the di-
                           fered from E. coli poisoning, and one person died. All of               The routes of infection suggest the tactics for self-
                                                                                                                                                                                                                           Calicivirus or Norwalk-like virus, which causes vomiting
                           these cases have been traced to tainted spinach                    defense against E. coli O157:H7. While preparing meat,
                                                                                                                                                             gestive system. There are almost as many food-borne           that lasts for approximately two days, with little diar-
                           crops,causing a minor crisis in the spinach and lettuce in-        segregate it from other food, clean up carefully, and wash     diseases as there are foods to carr y them. More than         rhea or fever. Norwalk-like virus has even spread from
                           dustry. What exactly is it that causes these problems? How         hands often. Cook hamburger to at least 72°C (160°F).          250 food-borne diseases are known, ranging from bacteria      infected fishermen through their oyster catch. Stomach
                           do the bacteria get into our food supply in the first place?       If you cannot check the temperature with a digital ther-       and viruses to parasites and toxins from the foods them-
                                 Bacteria are divided into “strains” according to some        mometer, cook until the pink inside turns brown. If children                                                                 flu has similar symptoms; it is actually not influenza but
                           definable genetic trait, and one of the many strains of E.         develop this infection, they must be especially careful to     selves. The many types of food poisoning share a com-         rather a viral infection that attacks and irritates the
                           coli causes a severe form of food poisoning. Called E. coli        observe sanitary procedures, especially frequent               mon thread. They are usually found growing in or on           stomach and small intestine. Stomach flu is transmitted
                           O157:H7, this genetic variant releases deadly toxins that          hand-washing, so they do not pass it along.                    the foods we eat. All of these diseases enter the body        through kissing, touching, or sharing food, drinks, or
                           cause severe, bloody diar-
                                                                                                                                                             through the digestive tract. Symptoms can vary, but the       utensils. Food preparation workers who carry the virus
                           rhea. The Centers for Dis-
                           ease Control and Prevention                                                                                                       immediate symptoms usually include nausea, vomiting,          can spread it through the food.
                           estimates that E. coli                                                                                                            abdominal cramps, and/or diarrhea. These symptoms                       Adding nutrients to the blood changes the
                           O157:H7 causes about                                                                                                              represent the body’s attempt to rid itself of the
                           73,000 illnesses per year.
                                                                                                                                                                                                                           blood chemistry, and digesting that pizza has pulled wa-
                           Most infections usually
                                                                                                                                                             pathogen or toxin. If these flushing techniques fail, we      ter from body fluids. These changes must be rectified
                           clear up by themselves after                                                                                                      will experience the specific symptoms of the invading         to keep the blood and other body fluids within their
                           5 to 10 days. Antibiotics are                                                                                                     organism.                                                     narrow ranges. Maintaining fluid homeostasis is a mat-
                           not needed and may con-                                                                                                                    Three common bacterial food poisonings are
                           tribute to kidney damage.                                                                                                                                                                       ter of survival. Monitoring and maintaining the compo-
                                 In rare cases, E. coli                                                                                                      Campylobacter, Salmonella, and Escherichia coli (E. coli).    sition of the blood and the entire internal environment
                           O157:H7 can cause a far                                                                                                           Campylobacter is a normal resident of the intestinal tract    is the job of yet another system, the urinar y system,
                           more serious disease, called                                                                                                      of chickens and other fowl. Commonly ingested in un-          which is covered in Chapter 14.
                           hemolytic uremic syn-
                                                                                                                                                             dercooked poultr y, Campylobacter is the number one
                           drome. This syndrome kills
                           red blood cells and causes
                           an average of 61 fatalities
                           each year in the United
                           States, mainly through                                                                                                                                                 CONCEPT CHECK
                           kidney failure. The syn-
                           drome is most severe                                                                                                                                                     List and briefly describe      How are
                           among children, the elderly,                                                                                                                                                the three most common          Campylobacter and
                           and people with immune
                                                                                                                                                                                                       eating disorders.              E. coli related? Where
                           deficiencies. Some survivors
                                                                                                                                                                                                                                      are they usually
                           require dialysis, others can
                           suffer seizures or paralysis.                                                                                                                                                                              found? What do they
                                                                                                                                                                                                                                      cause if ingested?




                    www.wiley.com/
                    college/ireland




                    438               CHAPTER 13 The Digestive System                                                                                                                                   Nutritional Health and Eating Disorders: You Truly are What You Eat     439
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           CHAPTER SUMMARY
                 Nutrients Are Life-                                The Digestive System
            1    sustaining                                   2     Processes Food from Start
                                                                    to Finish
                                                                                                                                                             4   Nutritional Health and Eating Disorders:
                                                                                                                                                                 You Truly Are What You Eat
                 Food contains macronutrients: carbo-                                                                                                             The primary nutritional disease in the United States is proba-
            hydrates, fats, and proteins; and micronutri-          The digestive system ingests food,        stomach, small intestine, large intestine,      bly obesity. The major eating disorders are anorexia nervosa, bu-
            ents: vitamins and minerals. Vitamins are         mixes and propels that food through the di-    and rectum. Accessory organs, including         limia nervosa, and binge eating disorder. All can be treated with a
            organic substances and minerals are inor-         gestive organs, mechanically and chemi-        the salivary glands, liver, gall bladder, and   combination of proper diet and professional mental health care. A
            ganic. Both are necessary for maintaining         cally breaks down the food, absorbs the nu-    pancreas, assist in digestion. The salivary     number of food-borne pathogens, both bacterial and viral, can
            homeostasis, and both can be obtained             trients from the food, and releases the        glands release salivary amylase and lubri-      cause disease, but good sanitation can prevent many of them from
            safely from over-the-counter supplements,         undigested wastes. The digestive system,       cate the bolus of food. The liver cleanses      being spread.
            but the daily diet should be rich in fruits and   or GI tract, is composed of one continuous     the blood as it drains from the small intes-
            vegetables. How much and what type of             tube, divided by sphincter muscles. Each or-   tine. The gall bladder stores and releases
            food we ingest plays a large role in our          gan has anatomical alterations that allow it   bile. The pancreas produces digestive en-
            health. The U.S. Food and Drug Administra-        to perform a specific function. The organs,    zymes and buffers that control the pH of the
            tion has recently upgraded the basic food
            pyramid to factor in age, activity levels, and
                                                              in order, are the oral cavity, esophagus,      digesting chyme in the small intestine.         KEY TERMS
            gender.
                                                                                                                                                             ■ aerobic p. 000                                ■ chyme p. 000                                 ■ milled p. 000
                                                                                                                                                             ■ amoeba p. 000                                 ■ fundus p. 000                                ■ nutrients p. 000
                                                                                                                                                             ■ apical membrane p. 000                        ■ gangrene p. 000                              ■ obligate anaerobes p. 000
                                                                                                                                                             ■ bacteriolytic p. 000                          ■ gastric p. 000                               ■ pancreatic Juice p. 000
                                                                                                                                                             ■ bolus p. 000                                  ■ glycolysis p. 000                            ■ papilla p. 000
                                                                                                                                                             ■ calories p. 000                               ■ hepatocytes p. 000                           ■ peristaltic wave p. 000
                                                                                                                                                             ■ chemical digestion p. 000                     ■ macerated p. 000                             ■ polyps p. 000
                                                                                                                                                             ■ chemiosmosis p. 000                           ■ mechanical digestion p. 000                  ■ vegan p. 000
                                                                                                                                                             ■ chylomicrons p. 000                           ■ mesenteries p. 000




                                                                                                                                                             CRITICAL THINKING QUESTIONS
                                                                                                                                                              1. Go to the MyPyramid Web site (http://www.mypyramid.gov/)            3. One of the more drastic solutions for overeating is to “staple”
                                                                                                                                                                 and obtain your personal food guide. Alter your personal char-         the stomach a procedure called gastric bypass surgery. This
                 Digestion Is Both                                                                                                                               acteristics, and compare the results. Describe what happens to         surgery reduces stomach size, preventing it from holding so
           3     Mechanical and Chemical                                                                                                                         the recommended guidelines as you age. What happens as the
                                                                                                                                                                 exercise level increases? Are these changes the same for males
                                                                                                                                                                                                                                        much. How would this affect the functioning of the stomach?
                                                                                                                                                                                                                                        What essential hormone will decrease in the blood as the sur-
                Mechanical digestion starts in the            down the material called chyme.                                                                    and females, or does gender radically alter the caloric recom-         face area of the stomach decreases?
           mouth, where the teeth grind and crush the         Macromolecules are absorbed                                                                        mendation?
           food. Saliva moistens the food, forming a          through the highly convoluted lining                                                                                                                                   4. Give a brief review of the structure of a liver lobule. Explain why
           bolus that can be swallowed. Muscular con-         of the small intestine and into the                                                             2. Starting at the esophagus, trace the pathway of food through           cirrhosis of the liver can lead to jaundice and eventual liver fail-
           tractions push the bolus through the esoph-        blood supply. As the now-nutrient de-                                                              the system. At each organ, indicate what anatomical adapta-            ure. What exactly prevents the liver lobule from functioning?
           agus into the stomach, where high acidity          pleted chyme moves through the                                                                     tions have been made to the general GI tract tube structure that
                                                                                                                                                                 allow for the specific functions of that organ.                     5. Some emotions curb the appetite. How does this happen?
           starts to break it down. This acidity kills        large intestine, water is removed. The
           most pathogens but can attack the stomach          waste material, including a large pro-
           wall if the mucus lining is damaged. In the        portion of harmless bacteria, is
           small intestine, enzymes continue to break-        moved into the rectum and excreted.




           440      CHAPTER 13 The Digestive System                                                                                                                                                                                                             Critical Thinking Questions            441
human_ch13_404-443v2.qxd       24-01-2007       16:28        Page 442




           SELF TEST
            1. Macronutrients include all of the following EXCEPT                                                        6. True or false: Calcium, zinc and iodine are all examples of         12. One function of the organ containing                            17. The structure shown below is found in the __________ and
               a.    carbohydrates.                                                                                         vitamins.                                                               these structures is                                                 serves to __________.
               b.    lipids.                                                                                                                                                                        a. chemical digestion                                               a.   large intestine, decrease surface area
                                                                                                                         7. The correct order of layers in the GI tract from external surface
               c.    vitamins.                                                                                                                                                                         of carbohydrates.                                                b.   small intestine, increase surface area
                                                                                                                            to lumen is
               d.    proteins.                                                                                                                                                                      b. mechanical                                                       c.   stomach, produce HCl.
                                                                            Glucose
                                                                                                                            a.   serosa S muscularis S submucosa S mucosa                              digestive action                                                 d.   liver, produce and store bile
            2. The reactions in this diagram are                                             ATP
                                                                                                                            b.   mucosa S submucosa S serosa S muscularis                              of bile.
               collectively referred to as                                                                                  c.   muscularis S submucosa S mucosa S serosa                           c. chemical digestion
                                                                           Pyruvic acid


               a.    chemiosmosis.                                                                      Mitochondrion       d.   submucosa S mucosa S muscularis S serosa                              of proteins.
                                                        Mitochondrial
                                                        matrix

               b.    the Krebs cycle.                                                                                    8. The muscularis of the GI tract is responsible for                       d. nutrient
                                                                                     CO2
               c.    mitochondrial reactions.                                                                                                                                                          absorption.
                                                                                     NAD+
                                                                                                                            a.   protecting the lumen.
               d.    cellular respiration.                                            NADH + H+

                                                                        Acetyl Coenzyme A
                                                                                                                            b.   creating the peristaltic wave.
                                                                                                                            c.   absorbing water and nutrients.
                                                                                          CO2                               d.   allowing the tract to slide around inside the abdominal
                                                                                                +
                                                                                                                                 cavity.
                                                                                    NADH + H

                                                                                    FADH 2
                                                                                                                         9. The teeth responsible for grinding and crushing are the
                                                                                                                            a.   incisors.
                                                                                                                                                                                                13. The organ that is responsible for producing digestive
                                                    O2                                                                      b.   canines.
                                                                                                                                                                                                    enzymes is the
                                                                                                                            c.   premolars.
                                                                              ATP                                           d.   All types of teeth grind food.                                     a.   liver.
                                                                               H2O                                                                                                                  b.   gall bladder.
                                                                                                                        10. Immune defenses in the digestive system include all of the fol-         c.   pancreas.
                                                                                                                            lowing EXCEPT                                                           d.   sublingual salivary gland.
            3. The first step in the reaction shown above                                                                   a.   MALT.
                                                                                                                                                                                                14. Most stomach ulcers are caused by
               a.    is called glycolysis.                                                                                  b.   Peyers patches.
               b.    converts one glucose molecule to two pyruvate molecules.                                               c.   liver.                                                             a.   stress.                                                    18. The most common viral liver disease in the United States is
               c.    releases a net of 2 ATP molecules.                                                                     d.   tonsils.                                                           b.   aspirin eroding the mucus lining of the stomach.               a.   hepatitis A.
               d.    all of the above describe the first reaction shown.                                                                                                                            c.   a spiral bacterium.                                            b.   hepatitis B.
                                                                                                                        11. The stage of swallowing that involves the rising of the larynx is       d.   alcoholism.                                                    c.   hepatitis C.
            4. In the figure below, the molecule of unsaturated fat is                                                      shown in this figure as                                                                                                                     d.   All three are equally uncommon.
               indicated as                                                                                                                                                                     15. The phase of gastric digestion that is initiated simply by
                                                                                                                            a. A
                                                                                                                                                                                                    the smell of food is the                                        19. The common eating disorder, anorexia nervosa, can be de-
               a.    A                                                                                                      b. B
                                                                                                                                                                                                    a.   cephalic phase.                                                scribed as
               b.    B
               c.    Neither of these molecules are unsaturated fats.                                                                                                                               b.   gastric phase.                                                 a.   the binge-purge disease.
               d.    Both of these molecules are unsaturated fats.                                                                                                                                  c.   intestinal phase.                                              b.   food poisoning.
                                                                                                                                                                                                    d.   All three phases are triggered by the smell of food.           c.   overeating.
                        H H H H H H H H H H H H H H H                               O                                                                                                                                                                                   d.   severe under-eating.
                 A    H C C C C C C C C C C C C C C C C                                                                                                                                         16. The function of the organ containing the structures shown be-
                                                                                    OH
                        H H H H H H H H H H H H H H H
                                                                                                                                                                                                    low is to                                                       20. The bacterium E. coli is normally found
                                                                                                                                                          Bolus
                                                                                                                                                                                                    a.   chemically digest food.                                        a.   in the colon.
                        H H H H H H H H            H H H H H H H                                   O                                                      Tongue
                 B    H C C C C C C C C C C C C C C C C C C                                                                                                                                         b.   mechanically digest food.                                      b.   in the small intestine.
                                                                                                   OH                                                                                                                                                                   c.   in the stomach.
                        H H H H H H H H H H H H H H H H H                                                                                                                                           c.   absorb nutrients.
                                                                                                                                                                                                    d.   All of the above are true of this organ.                       d.   throughout the digestive system.
            5. The myPyramid website is designed to give you
               a. tips on healthy eating in general.
               b. easy access to the caloric content of most common foods.
               c. tips on healthy eating based on your gender, age and activ-                                                     A                                      B
                  ity level.
               d. assistance in reducing obesity.



           442       CHAPTER 13 The Digestive System                                                                                                                                                                                                                                                                  Self Test       443

				
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