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The Digestive CHAPTER OUTLINE
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.
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Glycolysis, the Krebs cycle, and
Nutrients are Life-Sustaining electron transport Figure 13.1
In cytosol Glycolysis ATP
(vitamins and minerals). These are organic and inor- Glycolysis occurs in the cytoplasm, requiring two molecules
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-
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
NADH + H+
Aerobic another 6 ATPs in the electron transport chain.
compounds and must obtain The average supermarket contains more than 20,000
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.
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
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/
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-
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
• 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
• 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
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-
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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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.
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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
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.
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.
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
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)
THE GI TRACT REMAINS THE SAME
THROUGHOUT ITS LENGTH Mouth (oral cavity)
The digestive system is sometimes called a “tube within Parotid gland
a tube,” because it is a hollow structure with two open- (salivary gland)
ings that runs the length of your body. The digestive Submandibular gland Pharynx
system, also called the “gastrointestinal system” or GI SUBMUCOSA
tract, begins at the oral cavity, winds through the ab-
dominal cavity, and ends at the anus (Figure 13.4). MUSCULARIS:
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
the tract, the mucularis is composed of one layer of lon- Descending
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
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
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
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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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.
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
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.
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-
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-
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
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
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-
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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
vomiting to help the specific immune system rid the
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.
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
this organ, which ends with the lower esophageal sphincter. Serosa
BODY Longitudinal layer
curvature Circular layer
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/
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
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
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
ing short fatty acids such as those found in milk, but ● Intestinal phase. In the final phase of gastric
the stomach. digestion, the chyme begins to leave through the
works at an optimum pH of 5 or 6. In adults, both gas-
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
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
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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
the lacteal (Figure 13.18). large protein to carry a small fat. HDL carries small fats
Absorbed triglycerides are too from storage to the muscles and liver, where they are
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-
verted to chylomicrons and sess cholesterol levels and heart disease risk.
to the liver.
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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,
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
13.20), designed to allow Hepatitis B is passed from person to person in swollen due to infection. In either instance, the con-
maximum contact between he- body fluids. This form of hepatitis is serious because it tents of the appendix cannot move, leading to a
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-
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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
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
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
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
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
Most of our digestive enzymes are
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 binds to food (substrate)
2 Pancreatic amylase Pancreatic acinar cells. Starches (polysaccharides). Maltose (disaccharide),
2 maltotriose (trisaccharide), and
● 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
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.
Dipeptidase Small intestine. Dipeptides. Amino acids.
All digestive enzymes with the exception of two
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
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-
• 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?
438 CHAPTER 13 The Digestive System Nutritional Health and Eating Disorders: You Truly are What You Eat 439
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Nutrients Are Life- The Digestive System
1 sustaining 2 Processes Food from Start
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
■ 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
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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.
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
a. chemiosmosis. Mitochondrion d. submucosa S mucosa S muscularis S serosa of proteins.
b. the Krebs cycle. 8. The muscularis of the GI tract is responsible for d. nutrient
c. mitochondrial reactions. absorption.
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
NADH + H
9. The teeth responsible for grinding and crushing are the
13. The organ that is responsible for producing digestive
O2 b. canines.
enzymes is the
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
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
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-
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
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
d. assistance in reducing obesity.
442 CHAPTER 13 The Digestive System Self Test 443