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                                                                                                                                  C H A P T E R



                                                                                                                                                      8
                                                                                                                         Digestive System
                                                                                                                            and Nutrition


               N      ow weighing more than 350 pounds,
                      17-year-old Monica sat on the edge
               of her bed, staring at her reflection in the
                                                                    C    H   A    P   T    E   R

                                                                        8.1 Overview of Digestion
                                                                                                       C   O    N    C    E   P    T   S



               mirror on the wall. Disgusted with what she                   The gastrointestinal tract (GI tract) contains a number of organs that carry out
               saw, she mulled her options silently in her                   several processes as they digest food. The wall of the tract typically has four layers
               head. She had tried dieting—Jenny Craig,                      and is modified in each of the organs.
               Weight Watchers, the Atkins diet, and many
                                                                        8.2 First Part of the Digestive Tract
               others—and had lost a few pounds here and
                                                                             The mouth receives food where it is chewed as chemical digestion begins before
               there, but the weight loss never seemed to                    food enters the pharynx and the esophagus, which takes it to the stomach.
               last. She had even consulted a nutritionist,
               to no avail. Unable to control her weight by             8.3 The Stomach and Small Intestine
               any other means, she decided to ask her doc-                  The stomach stores food and continues chemical digestion, which is completed in
               tor about surgical remedies.                                  the small intestine. The products of digestion are absorbed by the small intestine
                                                                             into the blood or into the lymph.
                     Monica is considering bariatric surgery,
               a term used to describe several procedures               8.4 Three Accessory Organs and Regulation of Secretions
               that are performed on the digestive system                    The pancreas produces pancreatic juice, which is sent to the small intestine for the
               to promote weight loss. One common pro-                       chemical digestion of food, and the hormone insulin, which ordinarily causes cells,
               cedure is known as “stomach stapling.” In                     including liver cells, to take up glucose from the blood. The liver is a major metabolic
                                                                             organ, which stores glucose as glycogen and produces bile. Bile is stored in the
               this procedure, the stomach is reduced to
                                                                             gallbladder before it is sent to the small intestine for emulsification of fats.
               the size of a golf ball, and the digestive tract
               is rerouted so that food bypasses the first               8.5 The Large Intestine and Defecation
               two feet of the intestine. Bariatric surgery                  The large intestine absorbs water and vitamins produced by bacteria that normally
               compels patients to eat less because they be-                 reside in the large intestine, and it carries out defecation.
               come very ill if they overeat. After bariatric
                                                                        8.6 Nutrition and Weight Control
               surgery is performed, patients remain at a
                                                                             Planning nutritious meals and snacks to control one’s weight involves making
               lifelong risk of nutritional deficiencies.                     healthy and informed food choices.
                     With nearly two-thirds of Americans
               overweight and one-third obese, we are
               mired in an obesity epidemic. Bariatric sur-
               gery is becoming more widely used, and an
               increasing number of obese teens, like Mon-
               ica, are asking for these procedures. As you
               read this chapter, you will learn how the di-
               gestive system is organized, how it ordinar-
               ily works, and how its function is regulated
               by other organs of the body.
                                                                                                                                                                 143



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       144        Part II        Maintenance of the Human Body


                                                                           longer, more active and productive life. Unfortunately,
       8.1 Overview of Digestion                                           poor diet and lack of physical activity now rivals smoking
       The organs of the digestive system are located within a tube        as a major cause of preventable death in the United States.
       called the gastrointestinal tract (GI tract), which is depicted   • Digestion can be mechanical or chemical. Mechanical
       in Figure 8.1. Food, whether it is a Big Mac or a shrimp salad,     digestion occurs when food is divided into pieces that
       consists of the organic macromolecules you studied in Chap-         can be acted on by the digestive enzymes. Actually,
       ter 2, namely carbohydrates, fats, and proteins. These mole-        we aid mechanical digestion when we cut up our
       cules are too big to cross plasma membranes, and the purpose        food prior to ingestion. Mechanical digestion occurs
       of digestion is to hydrolyze these macromolecules to their          primarily in the mouth and stomach.
       unit molecules. The unit molecules, namely sugars, amino                 All parts of the tract, except the large intestine,
       acids, fatty acids, and glycerol, can cross plasma membranes.       contain digestive enzymes that hydrolyze particular
       These are the nutrients that are carried by the blood to our        foods to molecular nutrients. Chemical digestion begins
       cells. Our food also contains water, salts, vitamins, and min-      in the mouth and is not completed until food reaches
       erals that help the body function normally. Specifically, these      the small intestine. The thick semifluid mass of partly
       processes are necessary to the digestive process.                   digested food that is passed from the stomach to the
         • Ingestion occurs when the mouth takes in food. Ingestion        small intestine is termed chyme.
           can be associated with our diet. The expression “we           • Movement of GI tract contents along the digestive
           dig our graves with our teeth” recognizes that                  tract is very important in order for the tract to fulfill its
           our diet is very important to our health.                         other functions. For example, food must be passed
           By learning about good nutrition, you                                along from one organ to the next, and indigestible
           increase your likelihood of enjoying a                                 remains must be expelled.




        Accessory organs                                                                            Digestive tract organs
                                                                                                    Mouth
       Salivary glands                                                                              teeth chew food; tongue tastes and
       secrete saliva: contains digestive enzyme                                                    pushes food for chewing and swallowing
       for carbohydrates
                                                                                                    Pharynx
                                                                                                    passageway where food is swallowed




                                                                                                    Esophagus
                                                                                                    passageway where peristalsis pushes
                                                                                                    food to stomach




       Liver
       major metabolic organ:                                                                       Stomach
       processes and stores nutrients;                                                              secretes acid and digestive enzyme
       produces bile for emulsification of fats                                                     for protein; churns mixing food with
                                                                                                    secretions and sends chyme to
       Gall bladder                                                                                 small intestine
       stores bile from liver; sends it to
       the small intestine
                                                                                                    Small intestine
       Pancreas                                                                                     mixes chyme with digestive enzymes
       produces pancreatic juice: contains digestive                                                for final breakdown; absorbs nutrient
       enzymes, and sends it to the small intestine;                                                molecules into body; secretes
       produces insulin and secretes it into the                                                    digestive hormones into blood
       blood after eating

                                                                                                    Large intestine
                                                                                                    absorbs water and salt to form feces
       Figure 8.1
       The human gastrointestinal (GI) tract.                                                       Rectum
       Food is digested within the organs of the                                                    stores and regulates elimination
                                                                                                    of feces
       tract; the accessory organs assist this
       process in the ways noted.                                                                   Anus




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                                                                                                 Chapter 8       Digestive System and Nutrition              145

                                                                                       nerve     lymph
               Mucosa                                                                  supply    vessel
               inner mucous membrane
               layer that is modified
               according to the
               digestive organ



               Submucosa
               broad band of loose
               connective tissue that
               contains nerves, blood,
               and lymphatic vessels
                                                                                          lumen

                                                                                          artery
               Muscularis                                                                 vein
               two layers of
               smooth muscle
               Serosa                                                                                     Figure 8.2 Wall of the gastrointestinal tract.
               thin outermost tissue                                                                      The wall of the gastrointestinal tract contains the four
               that is the visceral                                                                       layers noted.
               peritoneum



                                                                                       The second layer in the GI wall is called the submucosa.
                                                                                  The submucosal layer is a broad band of loose connective tis-
                                                                                  sue that contains blood vessels, lymphatic vessels, and nerves.
                                                                                  These are the vessels that will carry the nutrients absorbed by
                                                                                  the mucosa. Lymph nodules, called Peyer’s patches, are also
                 • Absorption occurs as unit molecules produced by                in the submucosa. Like the tonsils, they help protect us from
                   digestion (i.e., nutrients) cross the wall of the GI tract     disease. Because the submucosa contains blood vessels, it can
                   and enter the cells lining the tract. From there, the          be the site of an inflammatory response (see page 128) that
                   nutrients enter the blood for delivery to the cells.           leads to inflammatory bowel disease (IBD), characterized by
                 • Elimination: Molecules that cannot be digested                 chronic diarrhea, abdominal pain, fever, and weight loss.
                   need to be eliminated from the body. The removal of                 The third layer is termed the muscularis, and it contains
                   indigestible wastes through the anus, in the form of           two layers of smooth muscle. The inner, circular layer encir-
                   feces, is defecation.                                          cles the tract; the outer, longitudinal layer lies in the same
                                                                                  direction as the tract. The contraction of these muscles, which
               Wall of the Digestive Tract                                        are under nervous control, accounts for movement of digested
                                                                                  food from the esophagus to the anus. The muscularis can be
               We can compare the GI tract to a garden hose that has a            associated with irritable bowel syndrome (IBS), in which con-
               beginning (mouth) and an end (anus). The so-called lumen           tractions of the wall cause abdominal pain, constipation,
               is the central space that contains water (food being digested).    and/or diarrhea. The underlying cause of IBS is not known,
               The wall of the GI tract has four layers (Fig. 8.2), and we will   although some suggest stress as an underlying cause.
               associate each layer with a particular disorder.                        The fourth layer of the tract is the serosa (serous mem-
                    The first layer of the wall next to the lumen is called the    brane layer), which secrete a serous fluid. The serosa is a part
               mucosa. The mucosa is more familiarly called a mucous              of the peritoneum, the internal lining of the abdominal cavity.
               membrane, and, of course, it produces mucus, which pro-            The appendix is a worm-shaped blind tube projecting from
               tects the wall from the digestive enzymes inside the lumen.        the first part of the large intestine on the right side of the abdo-
               In the mouth, stomach, and small intestine, the mucosa             men. An inflamed appendix (appendicitis) has to be removed
               either contains glands that secrete and/or receive digestive       because should the appendix burst, the result can be peritoni-
               enzymes from glands that secrete digestive enzymes.                tis, a life-threatening infection of the peritoneum.
                    Diverticulosis is a condition in which portions of the
               mucosa literally have pushed through the other layers and
               formed pouches, where food can collect. The pouches can be             Check Your Progress 8.1
               likened to an inner tube that pokes through weak places in a        1. Name and describe the processes that occur during the digestive
               tire. When the pouches become infected or inflamed, the                 process.
               condition is called diverticulitis. This happens in 10–25% of       2. What are the four layers of the GI tract? Associate an illness with
               people with diverticulosis.                                            each of the layers.




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       146        Part II        Maintenance of the Human Body


       8.2 First Part of the Digestive Tract                              The Teeth and Tongue
                                                                          Mechanical digestion occurs when our teeth chew food into
       The mouth, the pharynx, and the esophagus are in the first          pieces convenient for swallowing. Monica, in the opening
       part of the GI tract.                                              story, will have to chew her food thoroughly after her opera-
                                                                          tion because her stomach will no longer be able to continue
       The Mouth                                                          the process of mechanical digestion. Monica might even have
       The mouth receives food and begins the process of mechani-         to put her food in a blender before eating it.
       cal and chemical digestion. The mouth is bounded externally              During the first two years of life, the 20 smaller decidu-
       by the lips and cheeks. The lips extend from the base of the       ous, or baby, teeth appear. These are eventually replaced by
       nose to the start of the chin. The red portion of the lips is      32 adult teeth (Fig. 8.3a). The third pair of molars, called the
       poorly keratinized, and this allows blood to show through.         wisdom teeth, sometimes fail to erupt. If they push on the
            The roof of the mouth separates the nasal cavities from       other teeth and/or cause pain, they can be removed by a den-
       the oral cavity. The roof has two parts: an anterior (toward       tist or oral surgeon. Each tooth has two main divisions: a
       the front) hard palate and a posterior (toward the back) soft      crown and a root (Fig. 8.3b). The crown has a layer of enamel,
       palate (Fig. 8.3a). The hard palate contains several bones,        an extremely hard outer covering of calcium compounds;
       but the soft palate is composed entirely of muscle. The soft       dentin, a thick layer of bonelike material; and an inner pulp,
       palate ends in a finger-shaped projection called the uvula.         which contains the nerves and the blood vessels. Dentin and
       The tonsils, which are lymphatic tissue and help protect us        pulp are also found in the root.
       from disease, are also in the back of the mouth, on either side          Tooth decay, called dental caries, or cavities, occurs
       of the tongue, and in the nasopharynx (called adenoids).           when bacteria within the mouth metabolize sugar and give
            Three pairs of salivary glands send juices (saliva) by way    off acids, which erode teeth. Tooth decay can be painful
       of ducts to the mouth. One pair of salivary glands lies at the     when it is severe enough to reach the nerves of the inner
       sides of the face immediately below and in front of the ears.      pulp. Two measures can prevent tooth decay: eating a lim-
       These glands swell when a person has the mumps, a disease          ited amount of sweets, and daily brushing and flossing of
       caused by a viral infection. Salivary glands have ducts that       teeth. Fluoride treatments, particularly in children, can make
       open on the inner surface of the cheek at the location of the      the enamel stronger and more resistant to decay. Gum dis-
       second upper molar. Another pair of salivary glands lies           ease, which is now known to be linked to cardiovascular
       beneath the tongue, and still another pair lies beneath the        disease, is more apt to occur with aging. Inflammation of the
       floor of the oral cavity. The ducts from these salivary glands      gums (gingivitis) can spread to the periodontal membrane,
       open under the tongue. You can locate the openings if you use      which lines the tooth socket. A person then has periodonti-
       your tongue to feel for small flaps on the inside of your cheek     tis, characterized by a loss of bone and loosening of the teeth
       and under your tongue. Saliva is a solution of mucus and           so that extensive dental work may be required. Stimulation
       water, which also contains bicarbonate and an enzyme called        of the gums in a manner advised by your dentist is helpful
       salivary amylase that begins the process of digesting starch.      in controlling this condition. Medications are also available.


       Figure 8.3 Adult mouth
       and teeth.                                                                                                              enamel
       a. The chisel-shaped incisors                                                     crown                                 dentin
       bite; the pointed canines tear;                                                                                         pulp
       the fairly flat premolars grind;                                   hard palate                                           gum
       and the flattened molars crush
       food. b. Longitudinal section of a
       tooth. The crown is the portion                                                                                         jawbone
                                                                         soft palate
       that projects above the gum line
       and can be replaced by a dentist                                  uvula                                                 root canal
       if damaged. When a “root canal”                                   tonsil
                                                                                          root
       is done, the nerves are removed.
       When the periodontal membrane                                                                                           periodontal
       is inflamed, the teeth can loosen.                                 molars (3)                                            membrane
                                                                         premolars (2)
                                                                                                                               cementum
                                                                         canine (1)
                                                                         incisors (2)

                                                a.                                          b.




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                                                                                                                    Chapter 8        Digestive System and Nutrition               147


                Voluntary phase                                           Involuntary phase
                      hard palate food bolus
                                                nasopharynx



                                                                                                                                                                              peristatic
                                                             soft                                                                                                              wave
                                                                                                                             bolus
                                                             palate


                                                             epiglottis


                                                             esophagus
                                 glottis                     is closed
                                                                                  larynx rises and
                                                             trachea              epiglottis covers                       esophagus        lower esophageal
                                                             (airway)open         glottis                                                  spincter relaxes and           stomach
                                                                                                                                           food enters stomach
                a. Swallowing                                                                                             b. Peristalsis

               Figure 8.4 Swallowing of food and peristalsis.
               a. When food is swallowed, the tongue pushes a bolus of food up against the soft palate (left). Then, the soft palate closes off the nasal cavities, and the
               epiglottis closes off the larynx so the bolus of food enters the esophagus (right). b. Peristalsis moves food through a sphincter into the stomach.



                   The tongue is covered by mucous membrane, which                                          Unfortunately, we have all had the unpleasant experience
               contains the sensory receptors called taste buds (see page                             of having food “go the wrong way.” The wrong way may be
               249). When taste buds are activated by the presence of food,                           either into the nasal cavities or into the trachea. If it is the lat-
               nerve impulses travel by way of nerves to the brain. The                               ter, coughing will most likely force the food up out of the tra-
               tongue is composed of skeletal muscle, and it assists the                              chea and into the pharynx again.
               teeth in carrying out mechanical digestion by moving food                                    A rhythmic contraction called peristalsis pushes the food
               around in the mouth. In preparation for swallowing, the                                along the esophagus and continues in all the organs of the
               tongue forms chewed food into a mass called a bolus that it                            digestive tract. The esophagus plays no role in the chemical
               pushes toward the pharynx.                                                             digestion of food. Its sole purpose is to move the food bolus
                                                                                                      from the mouth to the stomach. Sphincters are muscles that
                                                                                                      encircle tubes and act as valves; tubes close when sphincters
               The Pharynx and Esophagus                                                              contract, and they open when sphincters relax. The entrance
               Both the mouth and the nasal passages lead to a cavity called                          of the esophagus to the stomach is marked by a constriction,
               the pharynx (Fig. 8.4). The food passage and air passage                               often called a lower gastroesophageal sphincter, although the
               cross in the pharynx because the trachea (windpipe) is ante-                           muscle is not as developed as in a true sphincter. Relaxation
               rior to (in front of) the esophagus, a long, narrow tube that                          of the sphincter allows food to pass into the stomach, while
               takes food to the stomach.                                                             contraction prevents the acidic contents of the stomach from
                                                                                                      backing up into the esophagus. When food or saliva is swal-
               Swallowing                                                                             lowed, the sphincter relaxes for a few seconds to allow the
               Swallowing has a voluntary phase—from day-to-day liv-                                  food or saliva to pass from the esophagus into the stomach,
               ing, you know that you can swallow voluntarily. (Try it!)                              and then it closes again.
               However, once food or drink is pushed back into the phar-                                    Heartburn, discussed in the Health Focus on page 148,
               ynx, swallowing becomes a reflex action performed auto-                                 occurs due to acid reflux, when some of the stomach’s con-
               matically (without you willing it). During swallowing, food                            tents escape into the esophagus. When vomiting occurs, the
               normally enters the esophagus, a muscular tube that takes                              abdominal muscles and the diaphragm, a muscle that sepa-
               food to the stomach, because other possible avenues are                                rates the thoracic and abdominal cavities, contract.
               blocked. The soft palate moves back to close off the nasal
               passages, and the trachea moves up under the epiglottis to
               cover the glottis. The glottis is the opening to the larynx                                Check Your Progress 8.2
               (voice box), and therefore, the air passage. We do not breathe                          1. Describe the mechanical digestion and the chemical digestion
               when we swallow. The up-and-down movement of the                                           that occurs in the mouth.
               Adam’s apple, the front part of the larynx, is easy to observe                          2. What ordinarily prevents food from entering the nose or entering
               when a person swallows.                                                                    the trachea when you swallow?




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       148        Part II        Maintenance of the Human Body




             Health Focus
                                                       Hearburn (GERD)
          In the absence of other symptoms, that burning sensation in
          your chest may have nothing to do with your heart. Instead,
          it is likely due to acid reflux. The stomach contents are more
          acidic than those of the esophagus. When the stomach con-
          tents pass upward into the esophagus, the acidity begins to
          erode the lining of the esophagus, producing the burning sen-
          sation associated with heartburn. Sometimes the acidic fluid
          passes upward and is tasted in the back of the mouth, and then
          we say we have acid indigestion.
                Almost everyone has had acid reflux and heartburn at some
          time. The burning sensation occurs in the area of the esophagus
          that lies behind the heart, and that is why it is termed heart-
          burn. We might think it means we have a heart problem.            Figure 8A A possible candidate for acid reflux.
                When heartburn or acid reflux becomes a chronic condi-       Obesity, a rich diet, and inactivity makes one prone to GERD
          tion, the patient is diagnosed with gastroesophageal reflux        (gastroesophageal reflux disease).
          disease (GERD). The term signifies that the stomach (gastric
          refers to the stomach) and the esophagus are involved in the
                                                                                 • Avoid high-fat meals, such as those served by
          disease. In GERD, patients’ reflux is more frequent, remains
                                                                                   fast-food chains. Fatty foods stay in the stomach
          in the esophagus longer, and will often contain higher levels
                                                                                   longer and, therefore, cause the stomach to secrete
          of acid than in a patient with typical heartburn or acid re-
                                                                                   more acid.
          flux. People diagnosed with GERD may also experience pain
                                                                                 • Don’t overeat. In the movie Supersize Me, Morgan
          in their chest, feel like they are choking, and have trouble
                                                                                   Spurlock doesn’t just have acid reflux after eating
          swallowing. An anatomical disorder may be present. For
                                                                                   a meal consisting of a supersize hamburger, coke,
          example, normally during swallowing, wavelike contrac-
                                                                                   and fries. He throws up! Vomiting involves the same
          tions (peristalsis) of the esophagus will move food down
                                                                                   processes as does acid reflux.
          the esophagus and into the stomach. Patients with weak or
                                                                                 • Eat several small meals instead of three large meals
          abnormal esophageal contractions will have difficulty push-
                                                                                   a day. Avoid foods that lead to stomach acidity,
          ing food into the stomach. These weak contractions can also
                                                                                   including alcohol, any form of fat, cakes and candy,
          prevent reflux from being pushed back into the stomach after
                                                                                   tomato sauces, caffeinated beverages, and citrus
          it has entered the esophagus, thus producing GERD. When
                                                                                   fruits. Eat complex carbohydrates such as multigrain
          patients are lying down, the effects of abnormal esophageal
                                                                                   bread, brown rice, and pasta instead of foods high in
          contractions become more severe. This is because gravity is
                                                                                   refined sugar. Besides providing you with vitamins
          not helping to return reflux to the stomach.
                                                                                   and minerals, complex carbohydrates can help you
                Many people take over-the-counter medications for acid
                                                                                   control acid reflux!
          reflux. These drugs, being basic, neutralize stomach acid.
                                                                                 • Light exercise such as riding a bike at a slow pace,
          Some progress to taking prescribed medications, which shut
                                                                                   walking, yoga, and light weight lifting are usually
          down and reduce acid production. However, persons with
                                                                                   extremely helpful in heartburn patients. Both diet
          acid reflux could try modifying their eating habits first.
                                                                                   and exercise should help control weight.
                                                                                   The pressure applied to the abdominal wall from
          Diet and Exercise                                                        obesity places pressure on the stomach and causes
          It has been found that diet can help control acid reflux. Here            the normal acid contents of the stomach to reflux
          are some tips:                                                           upward into the esophagus.


                                                                            duodenum of the small intestine below. The stomach stores
       8.3 The Stomach and Small Intestine                                  food, initiates the digestion of protein, and controls the
       The stomach and small intestine complete the digestion of            movement of chyme into the small intestine. Notice that the
       food, which began in the mouth.                                      stomach does not absorb nutrients; however, it does absorb
                                                                            alcohol because alcohol is fat soluble and can pass through
                                                                            the membrane for that reason.
       The Stomach                                                              The stomach wall has the usual four layers, but two of
       The stomach (Fig. 8.5) is a thick-walled, J-shaped organ that        them are modified for particular functions. The muscularis
       lies on the left side of the body beneath the diaphragm. The         contains three layers of smooth muscle (Fig. 8.5a). In addi-
       stomach is continuous with the esophagus above and the               tion to the circular and longitudinal layers, the stomach also




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                                                                                                                  Chapter 8      Digestive System and Nutrition             149


               contains a layer of smooth muscle that runs obliquely to the                        sphincter, which closes and squeezes most of the chyme
               other two. The oblique layer also allows the stomach to                             back, letting only a small amount to enter the small intestine
               stretch and to mechanically break down food into smaller                            at one time (Fig. 8.5d).
               fragments that are mixed with gastric juice. Since Monica’s
               stomach will be the size of a golf ball after surgery, her stom-                    Historical Aspects
               ach will not be able to carry out mechanical digestion; there-                      The stomach is well known for providing new scientific
               fore, it will not be able to store food and she might be subject                    information. Not much was known, in general, about diges-
               to GERD, as described in the Health Focus on page 148.                              tion in 1822 when an opportunity arose for a surgeon, named
                    The mucosa of the stomach has deep folds, the rugae,                           William Beaumont, to study how the stomach functions.
               which disappear as the stomach fills to an approximate                               After a patient, Alexis St. Martin, was shot, leaving a hole in
               capacity of 1 liter. The mucosa of the stomach has millions of                      his stomach, Beaumont began performing experiments on
               gastric pits, which lead into gastric glands (Fig. 8.5b and c).                     digestion. Most of the experiments consisted of tying a piece
               The gastric glands produce gastric juice. Gastric juice con-                        of food to a string and inserting it through the hole in St.
               tains an enzyme called pepsin, which digests protein, plus                          Martin’s stomach. Every few hours, Beaumont would
               hydrochloric acid (HCl) and mucus. HCl causes the stomach                           remove the food and observe how well it had been digested.
               to have a high acidity with a pH of about 2, and this is ben-                       Through careful observations, he discovered most of what
               eficial because it kills most bacteria present in food. Although                     we know today about the stomach.
               HCl does not digest food, it does break down the connective                             Also, about a 150 years later, in 1982, Dr. Marshall and
               tissue of meat and activates pepsin.                                                Dr. Warren began their work on the cause of ulcers. As
                    Normally, the stomach empties in about 2–6 hours.                              related in Chapter 1, they discovered that the bacterium
               When food leaves the stomach, it is a thick, soupy liquid                           Heicobacter pyloris is able to overcome the protective function
               called chyme. Chyme enters the small intestine in squirts.                          of mucus in the stomach and the result is a gastric ulcer.
               The peristaltic waves move the chyme toward the pyloric                             (Gastric always refers to the stomach.)



                                                                               muscularis layer
                  lower gastroesophageal                                       has three layers
                  sphincter                                                    of muscle

                           pyloric                                             mucosa layer
                          sphincter                                            has rugae
                                 pyloric
                                 canal


                                                                                                   c. Gastric pits in mucosa                 gastric pit           SEM 3,260�




                                                                                                                                                     lower gastroesophageal
                                                                                                                                                     sphincter
              a. Stomach

                        gastric pit
                        gastric gland
                        cells that                                                                              pyloric
                        secrete                                                                                 sphincter
                        gastric
                        juice




                                      b. Gastric glands                                            d. How the stomach empties

               Figure 8.5 Structure and function of the stomach.
               a. Structure of the stomach showing that the muscularis has three muscle layers and the mucosa has folds called rugae. b. Gastric glands present in the mucosa
               secrete mucus, HCl, and pepsin, an enzyme that digests protein. c. Micrograph of gastric pits. d. Peristalsis in the stomach controls the secretion of chyme into
               the small intestine at the pyloric sphincter.




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       150        Part II        Maintenance of the Human Body


       The Small Intestine                                                      jections called villi (sing., villus), which give the intestinal
                                                                                wall a soft, velvety appearance (Fig. 8.6). A villus has an outer
       The small intestine is named for its small diameter (com-                layer of columnar epithelial cells, and each of these cells has
       pared with that of the large intestine), but perhaps it should           thousands of microscopic extensions called microvilli. Col-
       be called the long intestine. The small intestine averages               lectively, in electron micrographs, microvilli give the villi a
       about 6 m (18 ft) in length, compared with the large intes-              fuzzy border known as a “brush border.” Since the microvilli
       tine, which is about 1.5 m (4½ ft) in length.                            bear the intestinal enzymes, these enzymes are called brush-
                                                                                border enzymes. The microvilli greatly increase the surface
       Digestion Is Completed in the Small Intestine                            area of the villus for the absorption of nutrients.
       Notice in Table 8.1 that the small intestine contains enzymes                 Nutrients are absorbed into the vessels of a villus (Fig.
       to digest all types of foods, primarily carbohydrates, pro-              8.7). A villus contains blood capillaries and a small lymphatic
       teins, and fats. These enzymes are secreted by the pancreas              capillary, called a lacteal. As you know, the lymphatic system
       and enter via a duct at the duodenum, the name for the first              is an adjunct to the cardiovascular system; its vessels carry a
       25 cm of the small intestine. Also, a duct brings bile from the          fluid called lymph to the cardiovascular veins. Sugars
       liver and gallbladder into the duodenum (see Fig. 8.8). Bile             (digested from carbohydrates) and amino acids (digested
       emulsifies fat—emulsification causes fat droplets to disperse              from proteins) enter the blood capillaries of a villus. Glycerol
       in water. After fat is mechanically broken down to fat drop-             and fatty acids (digested from fats) enter the epithelial cells
       lets by bile, it is hydrolyzed to glycerol and fatty acids by            of the villi, and within these cells are joined and packaged as
       lipase present in pancreatic juice. Pancreatic amylase begins            lipoprotein droplets, called chylomicrons, which enter a lac-
       and an intestinal enzyme finishes the digestion of carbohy-               teal. After nutrients are absorbed, they are eventually carried
       drates to glucose. Similarly, pancreatic trypsin begins and an           to all the cells of the body by the bloodstream.
       intestinal enzyme finishes the digestion of proteins to amino
       acids. The intestine has a slightly basic pH because pancre-
       atic juice contains sodium bicarbonate (NaHCO3), which
                                                                                Lactose Intolerance
       neutralizes chyme. Monica’s surgery is associated with                   Lactose is the primary sugar in milk. People who do not have
       nutritional deficiencies because her GI tract now bypasses                the brush border enzyme called lactase can not digest lactose.
       the duodenum.                                                            The result is a condition called lactose intolerance, character-
                                                                                ized by diarrhea, gas, bloating, and abdominal cramps after
       Nutrients Are Absorbed in the Small Intestine                            drinking milk and other dairy products. Diarrhea occurs
       The wall of the small intestine absorbs the molecules, namely            because the indigestible lactose causes fluid retention in the
       sugars, amino acids, fatty acids, and glycerol, which are the            small intestine. Gas, bloating, and cramps occur when bacte-
       products of the digestive process. The wall of the small intes-          ria break down the lactose anaerobically, producing gas.
       tine is modified for absorption. It has been suggested that the                Persons with lactose intolerance can eat dairy products
       surface area of the small intestine is approximately that of a           such as cheese and yogurt in which the lactose has already
       tennis court. What factors contribute to increasing its surface          been broken down; lactose-free milk, and/or take a dietary
       area? The wall of the small intestine contains fingerlike pro-            supplement that aids in the digestion of lactose.



           Small intestine
                                       Section of intestinal wall                        villus

                                  lumen                                                 lacteal

                                                                                         blood                            villus    microvilli
                                                                                       capillaries

                                                                                         goblet
                                                                                          cell




                                                               lymph
                                                               nodule
       Figure 8.6 Anatomy of the small intestine.
                                                                                        venule
       The wall of the small intestine has folds that bear
       fingerlike projections called villi. The products of                             lymphatic
       digestion are absorbed by microvilli into the blood                               vessel
       capillaries and the lacteals of the villi.                       Villi           arteriole




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                 Table 8.1         Major Digestive Enzymes

                 Enzyme                            Produced By            Site of Action        Optimum pH            Digestion

                 CARBOHYDRATE DIGESTION:

                   Salivary amylase                Salivary glands        Mouth                 Neutral               Starch  H2O → maltose

                   Pancreatic amylase              Pancreas               Small intestine       Basic                 Starch  H2O → maltose

                   Maltase                         Small intestine        Small intestine       Basic                 Maltose  H2O → glucose  glucose
                 PROTEIN DIGESTION:

                   Pepsin                          Gastric glands         Stomach               Acidic                Protein  H2O → peptides

                   Trypsin                         Pancreas               Small intestine       Basic                 Protein  H2O → peptides

                   Peptidases                      Small intestine        Small intestine       Basic                 Peptide  H2O → amino acids
                 NUCLEIC ACID DIGESTION:

                   Nuclease                        Pancreas               Small intestine       Basic                 RNA and DNA  H2O → nucleotides

                   Nucleosidases                   Small intestine        Small intestine       Basic                 Nucleotide  H2O → base  sugar  phosphate
                 FAT DIGESTION:

                   Lipase                          Pancreas               Small intestine       Basic                 Fat droplet  H2O → monoglycerides  fatty acids




               Figure 8.7 Digestion and                                    carbohydrate                         protein
               absorption of nutrients.                                                                                                               +
               a. Starch is digested to glucose,              pancreatic amylase                                                         bile salts   fat globules
               which is actively transported into                                                         trypsin
               the epithelial cells of intestinal villi.                                                                                                  emulsification
                                                                                                                    peptides                              droplets
               From there, glucose moves into the                    maltase
                                                          epithelial                                         peptidase                           lipase
               bloodstream. b. Proteins are digested
                                                          cell of              glucose
               to amino acids, which are actively         intestinal                                                                                         monoglycerides
               transported into the epithelial cells of   villus                                  amino acids                                                and free
               intestinal villi. From there, amino acids                                                                                                     fatty acids
               move into the bloodstream. c. Fats
               are emulsified by bile and digested to
                                                                                             pH = basic                                 pH = basic               chylomicron
               monoglycerides and fatty acids. These pH = basic
               diffuse into epithelial cells, where they
               recombine and join with proteins to                                                                                      lymphatic
                                                                       blood capillary                     blood capillary
               form lipoproteins, called chylomicrons.                                                                                  capillary
               Chylomicrons enter a lacteal.             a. Carbohydrate digestion          b. Protein digestion                       c. Fat digestion



               Obesity: Diabetes Type 2 and                                                       Switching to a healthy diet, increasing physical activity,
                                                                                             and losing weight improves the ability of the hormone insulin
               Cardiovascular Disease                                                        to function properly in type 2 diabetics. The high fatty acid
               The nutrients absorbed at the small intestine have a profound                 levels that accompany diabetes type 2 can lead to an increased
               effect on the body because these molecules can affect our health.             risk of cardiovascular disease. It is well worthwhile to prevent
               For example, the intake of too much sugar and fat can result in               diabetes type 2 by switching to a healthy diet and engaging in
               obesity, which is associated with diabetes type 2 and cardiovas-              daily exercise because all diabetics are at risk for blindness,
               cular disease. Nutritionists point out that consuming too much                kidney disease, and cardiovascular disease.
               energy from any source contributes to body fat, which increases
               a person’s risk of obesity and associated illness. Still, foods such               Check Your Progress 8.3
               as doughnuts, cakes, pies, cookies, and white bread, which are                  1. a. What are the functions of the stomach, and (b) how is the wall
               high in refined carbohydrates (starches and sugars), and fried                      of the stomach modified to perform these functions?
               foods, which are high in fat, may very well be responsible for                  2. a. What are the functions of the small intestine, and (b) how is the
               the current epidemic of obesity among Americans.                                   wall of the small intestine modified to perform these functions?




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       152        Part II        Maintenance of the Human Body


                                                                                        blood via the hepatic portal vein from the capillary bed of the
       8.4 Three Accessory Organs and                                                   GI tract and filters blood in the capillaries of the lobules. In a
           Regulation of Secretions                                                     sense, the liver acts like a sewage treatment plant when it
       First, we will take a look at three accessory organs of diges-                   removes poisonous substances from the blood and detoxifies
       tion before considering how the secretions of these organs                       them, while blood courses through its lobules (Table 8.2).
       and those of the GI tract are regulated.                                              The liver is also a storage organ. It removes iron and the
                                                                                        vitamins A, D, E, K, and B12 from blood and stores them. After
                                                                                        we eat and in the presence of insulin, the liver stores glucose
       Three Accessory Organs                                                           as glycogen, and between eating, it breaks down glycogen to
       The pancreas is a fish-shaped, spongy, grayish pink organ                         keep the blood glucose level relatively constant. If need be,
       that stretches across the back of the abdomen behind the                         the liver converts glycerol (from fats) and amino acids to glu-
       stomach. Most pancreatic cells produce pancreatic juice,                         cose molecules. As amino acids are converted to glucose, the
       which enters the duodenum via the pancreatic duct (Fig. 8.8).                    liver combines their amino groups with carbon dioxide to
       Pancreatic juice contains sodium bicarbonate (NaHCO3) and                        form urea, the usual nitrogenous waste product in humans.
       digestive enzymes for all types of food. Sodium bicarbonate                           On the other hand, the liver makes the plasma proteins
       neutralizes acid chyme from the stomach. Pancreatic amylase                      and helps regulate the quantity of cholesterol in the blood. It
       digests starch, trypsin digests protein, and lipase digests fat.                 does this by producing bile salts, which are derived from
            The pancreas is also an endocrine gland that secretes the                   cholesterol. Bile has a yellowish green color because it also
       hormone insulin into the blood. A hormone is a substance pro-                    contains bilirubin, derived from the breakdown of hemoglo-
       duced by one set of cells that affects a different set of cells, the             bin, another function of the liver. Bile is stored in the gall-
       so-called target cells. When the blood glucose level rises rap-                  bladder, a pear-shaped organ just below the liver, until it is
       idly, the pancreas produces an overload of insulin to bring the                  sent via the bile ducts to the duodenum. Gallstones form
       level under control. Over the years, the body’s cells can become                 when liquid stored in the gallbladder hardens into pieces of
       insulin resistant with the result that diabetes type 2 can occur.                stonelike material. Monica may have a tendency toward
            The liver, which is the largest and a major metabolic                       gallstones after her surgery because her gallbladder is no
       gland in the body, lies mainly in the upper-right section of the                 longer ridding itself of bile. In the small intestine, bile salts
       abdominal cavity, under the diaphragm (see Fig. 8.1). The                        emulsify fat. When fat is emulsified, it breaks up into drop-
       liver contains approximately 100,000 lobules that serve as its                   lets, providing a much larger surface area, which can be
       structural and functional units (Fig. 8.8b). The liver receives                  acted upon by a digestive enzyme from the pancreas.



                                                                                                                                                             bile canals




                                                                                                                                                              branch of
                                                                                                                                                               hepatic
                                              bile                                                                                                              artery
                                                                                                  central
                                                                                                  vein



                                                     common hepatic duct
                                                                             pancreas
                                                      pancreatic duct



                                                                    pancreatic
                                                                    juice
                                                                                                                                                             branch of
       gallbladder                                                                                                                                            hepatic
                                                                                                                    b.           bile duct                   portal vein
       common bile duct

       duodenum
                                                                                        Figure 8.8 The liver.
                                                                                        a. The location of the liver, gallbladder, and pancreas in relation to the small
                                                                                        intestine. b. The liver contains over 100,000 lobules. Each lobule contains
       a.                                                                               many cells that perform the various functions of the liver (see Table 8.2).




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                                                                                                                 gallbladder         liver
                 Table 8.2          Functions of the Liver

                                                                                                                                                 stomach
                 1. Destroys old red blood cells; excretes bilirubin, a breakdown product of
                      hemoglobin in bile, a liver product
                 2. Detoxifies blood by removing and metabolizing poisonous substances
                 3. Stores iron (Fe2 +) and the fat-soluble vitamins A, D, E, and K
                 4. Makes plasma proteins, such as albumins and fibrinogen, from amino acids
                 5. Stores glucose as glycogen after a meal, and breaks down glycogen
                      to glucose to maintain the glucose concentration of blood between                                                              pancreas
                      eating periods
                 6. Produces urea after breaking down amino acids
                 7. Helps regulate blood cholesterol level, converting some to bile salts




               Liver Disorders                                                                                         duodenum                                   secretin
                                                                                                                                                                  CCK
               Hepatitis and cirrhosis are two serious diseases that affect                                                                                       gastrin
               the entire liver and hinder its ability to repair itself. There-                                       blood vessel
               fore, they are life-threatening diseases. When a person has
               a liver ailment, jaundice may occur in which bile pigments                      Figure 8.9 Hormonal control of digestive gland secretions.
               leak into the blood. Jaundice is a yellowish tint to the whites                 Gastrin (blue), from the lower stomach, feeds back to stimulate the upper
               of the eyes and also to the skin of light-pigmented persons.                    part of the stomach to produce digestive juice. Secretin (green) and CCK
               Jaundice can result from hepatitis, inflammation of the                          (purple) from the duodenal wall stimulate the pancreas to secrete digestive
               liver. Viral hepatitis occurs in several forms. Hepatitis A is                  juice and the gallbladder to release bile.
               usually acquired from sewage-contaminated drinking
               water. Hepatitis B, which is usually spread by sexual con-
               tact, can also be spread by blood transfusions or contami-
               nated needles. The hepatitis B virus is more contagious                         food, the parasympathetic nervous system automatically
               than the AIDS virus, which is spread in the same way. Vac-                      stimulates gastric secretion. Also, when a person has eaten a
               cines are now available for hepatitis A and hepatitis B.                        meal particularly rich in protein, the stomach produces the
               Hepatitis C, for which there is no vaccine, is usually                          hormone gastrin. Gastrin enters the bloodstream, and soon
               acquired by contact with infected blood and can lead to                         the secretory activity of gastric glands is increasing.
               chronic hepatitis, liver cancer, and death.                                          Cells of the duodenal wall produce two other hormones
                    Cirrhosis is another chronic disease of the liver. First, the              that are of particular interest—secretin and CCK (cholecys-
               organ becomes fatty, and then liver tissue is replaced by inac-                 tokinin). Acid, especially hydrochloric acid (HCl) present in
               tive fibrous scar tissue. Cirrhosis of the liver is often seen in                chyme, stimulates the release of secretin, while partially
               alcoholics, due to malnutrition and to the excessive amounts                    digested protein and fat stimulate the release of CCK. Soon
               of alcohol (a toxin) the liver is forced to break down. Physi-                  after these hormones enter the bloodstream, the pancreas
               cians are now beginning to see cirrhosis of the liver in obese                  increases its output of pancreatic juice. Pancreatic juice buf-
               people who are overweight due to a diet high in fatty foods.                    fers the acidic chyme entering the intestine from the stom-
                    The liver has amazing regenerative powers and can                          ach and helps digest food. CCK also causes the liver to
               recover if the rate of regeneration exceeds the rate of dam-                    increase its production of bile, and the gallbladder to con-
               age. During liver failure, however, there may not be enough                     tract and release stored bile. Figure 8.9 summarizes the
               time to let the liver heal itself. Liver transplantation is usu-                actions of gastrin, secretin, and CCK.
               ally the preferred treatment for liver failure, but artificial
               livers have been developed and tried in a few cases. The
               liver is a vital organ, and its failure leads to death.                             Check Your Progress 8.4
                                                                                                1. What are the three main accessory organs that assist with
                                                                                                   the digestive process?
               Regulation of Digestive Secretions                                               2. How does each accessory organ contribute to the digestion
               The secretions of digestive juices are controlled by the ner-                       of food?
               vous system and by hormones. When you look at or smell                           3. How are digestive secretions regulated in the body?




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       154        Part II        Maintenance of the Human Body


                                                                                          Escherichia coli were the major inhabitants of the colon, but
       8.5 The Large Intestine and                                                        new culture methods show that over 99% of the colon bacte-
           Defecation                                                                     ria are other types of bacteria. The bacteria in the large intes-
                                                                                          tine break down indigestible material, and also produce B
       The large intestine includes the cecum, the colon, the rectum,
                                                                                          complex vitamins and most of the vitamin K needed by our
       and the anal canal (Fig. 8.10). The large intestine is larger in
                                                                                          bodies. In this way, they perform a service for us.
       diameter than the small intestine (6.5 cm compared with 2.5
                                                                                               The large intestine forms feces. Feces are three-quarters
       cm), but it is shorter in length (see Fig. 8.1).
                                                                                          water and one-quarter solids. Bacteria, dietary fiber (indigest-
            The cecum, which lies below the junction with the small
                                                                                          ible remains), and other indigestible materials are in the solid
       intestine, is the blind end of the large intestine. The cecum
                                                                                          portion. Bacterial action on indigestible materials causes the
       usually has a small projection called the vermiform appen-
                                                                                          odor of feces and also accounts for the presence of gas. A
       dix (vermiform means wormlike) (Fig. 8.10). In humans, the
                                                                                          breakdown product of bilirubin (see page 152) and the pres-
       appendix also may play a role in fighting infections. As men-
                                                                                          ence of oxidized iron cause the brown color of feces.
       tioned previously, the appendix can become infected, a con-
                                                                                               Defecation, which is ridding the body of feces, is also a
       dition called appendicitis.
                                                                                          function of the large intestine. Peristalsis occurs infrequently
            The colon includes the ascending colon, which goes up
                                                                                          in the large intestine, but, when it does, feces are forced into
       the right side of the body to the level of the liver; the trans-
                                                                                          the rectum. Feces collect in the rectum until it is appropriate
       verse colon, which crosses the abdominal cavity just below
                                                                                          to defecate. At that time, stretching of the rectal wall initiates
       the liver and the stomach; the descending colon, which
                                                                                          nerve impulses to the spinal cord, and shortly thereafter, the
       passes down the left side of the body; and the sigmoid colon,
                                                                                          rectal muscles contract and the anal sphincters relax, allowing
       which enters the rectum, the last 20 cm of the large intestine.
                                                                                          the feces to exit the body through the anus (Fig. 8.10). A per-
       The rectum opens at the anus, where defecation, the expul-
                                                                                          son can inhibit defecation by contracting the external anal
       sion of feces, occurs.
                                                                                          sphincter. Ridding the body of indigestible remains is another
                                                                                          way the digestive system helps maintain homeostasis.
       Functions of the Large Intestine                                                        Water is considered unsafe for swimming when the coli-
                                                                                          form (nonpathogenic intestinal) bacterial count reaches a
       The large intestine absorbs water, which is an important func-
                                                                                          certain number. A high count indicates that a significant
       tion that prevents dehydration of the body. Note that the large
                                                                                          amount of feces has entered the water. The more feces pres-
       intestine does not produce any digestive enzymes and it does
                                                                                          ent, the greater the possibility that disease-causing bacteria
       not absorb any nutrients.
                                                                                          are also present.
            The large intestine absorbs vitamins produced by bacte-
       ria called the intestinal flora. For many years, it was believed
                                                                                          Disorders of the Colon and Rectum
                                                                                          The large intestine is subject to a number of disorders. Many
                                           transverse colon                               of these can be prevented or minimized by a good diet and
                                                                                          good bowel habits.
                                                                                          Diarrhea The major causes of diarrhea are infection of the
                                                                                          lower intestinal tract and nervous stimulation. In the case of
                                                                                          infection, such as food poisoning caused by eating contami-
         ascending                                                                        nated food, the intestinal wall becomes irritated, and peri-
         colon                          small         descending
                                      intestine       colon
                                                                                          stalsis increases. Water is not absorbed, and the diarrhea that
                                                                                          results rids the body of the infectious organisms. In nervous
                                                                                          diarrhea, the nervous system stimulates the intestinal wall,
                                                                                          and diarrhea results. Prolonged diarrhea can lead to dehydra-
         cecum                                                                            tion because of water loss and to disturbances in the heart’s
         orifice of
                                                                                          contraction due to an imbalance of salts in the blood.
         appendix
                                                                                          Constipation When a person is constipated, the feces are
         vermiform                                                                        dry and hard. One reason for this condition is that socialized
         appendix                     rectum
                                                                                          persons have learned to ignore the urge to defecate. An in-
                                                                                          take of water and fiber can help regularity of defecation. The
                      internal anal sphincter                 external anal sphincter     frequent use of laxatives is discouraged. If, however, it is nec-
                                  anal canal                  anus                        essary to take a laxative, a bulk laxative is the most natural
                                                                                          because, like fiber, it produces a soft mass of cellulose in the
       Figure 8.10 Large intestine.                                                       colon. Lubricants, such as mineral oil, make the colon slip-
       The regions of the large intestine are the cecum, colon, rectum, and anal canal.   pery; saline laxatives, such as milk of magnesia, act osmoti-




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                                                                                                  Chapter 8       Digestive System and Nutrition     155



                    Health Focus
                                                     Swallowing a Camera           create discomfort in the patient, the considerable risks in-
                                                                                   volved with using anesthetics and painkillers are elimi-
                  PillCam has become a viable alternative to traditional           nated. And finally, the doctor does not need to be present
                  endoscopy procedure, during which a doctor uses a re-            for the entire procedure, saving valuable time and money
                  tractable tubelike instrument with an embedded camera,           for both doctor and patient.
                  called an endoscope, to visualize the patient’s GI tract.             Colonoscopy is a routine procedure used to exam-
                  With a gulp of water, PillCam is swallowed, and its travels      ine and diagnose colon cancer. It employs a colonoscope,
                  through the digestive system begin. Instead of spending          an instrument similar to an endoscope, that is inserted
                  an uncomfortable half day or more at the doctor’s office, a       through the anus into the large intestine and is capable of
                  patient simply visits the doctor in the morning, swallows        removing precancerous tissue before it becomes invasive.
                  the camera, dons the recording device, and goes about his        Since PillCam cannot be used to remove tissue samples for
                  or her daily routine.                                            analysis, its use as a replacement for the colonoscopy is
                       Propelled by the normal muscular movement of the di-        somewhat limited.
                  gestive system, PillCam embarks on a four- to eight-hour
                  journey through the digestive system. As it travels through
                  the stomach, the twists and turns of the small intestine, and
                  the large intestine, PillCam continuously captures high-         Figure 8B Patient
                  quality, wide-angle film footage of its journey and continu-      swallowing PillCam.
                  ously beams this information to the recording device worn        PillCam traverses through the
                  by the patient.                                                  entire GI tract, taking pictures
                       At the end of the day, PillCam reaches the end of its       after it is swallowed.
                  journey, and it is simply defecated normally. Later, the re-
                  cording device is returned to the doctor’s office so that the
                  data may be retrieved. The doctor can view PillCam’s jour-
                  ney as a 90-minute movie.
                       Like the food that we eat, PillCam traverses the nu-
                  merous twists and turns of the intestine with ease. This
                  provides a more accurate diagnosis, since a larger portion
                  of the GI tract can be examined. Since PillCam does not



               cally—they prevent water from being absorbed. Some laxa-            layer, where nerves and blood vessels are. Colitis causes diar-
               tives are irritants that increase peristalsis.                      rhea, rectal bleeding, abdominal cramps, and urgency.
                    Chronic constipation is associated with the development
                                                                                   Polyps and Cancer The colon is subject to the development
               of hemorrhoids, enlarged and inflamed blood vessels at the
                                                                                   of polyps, small growths arising from the epithelial lining.
               anus. Other contributing factors include pregnancy, aging,
                                                                                   Polyps, whether benign or cancerous, can be removed surgi-
               and anal intercourse.
                                                                                   cally. If colon cancer is detected while still confined to a polyp,
               Diverticulosis As mentioned previously (see page 145),              the expected outcome is a complete cure. Some investigators
               diverticulosis is the occurrence of little pouches of mucosa        believe that dietary fat increases the likelihood of colon cancer
               that have pushed out through weak spots in the muscularis.          because dietary fat causes an increase in bile secretion. It could
               A frequent site is the last part of the descending colon.           be that intestinal bacteria convert bile salts to substances that
                                                                                   promote the development of cancer. On the other hand, fiber
               Irritable Bowel Syndrome (IBS) (spastic colon) Also men-
                                                                                   in the diet seems to inhibit the development of colon cancer.
               tioned previously (see page 145), IBS is a condition in which
                                                                                   Regular elimination reduces the time that the colon wall is
               the muscularis contracts powerfully but without its normal
                                                                                   exposed to any cancer-promoting agents in feces.
               coordination. The symptoms are abdominal cramps, gas,
                                                                                        As discussed in the Health Focus above, endoscopy by
               constipation, and urgent, explosive stools (feces discharge).
                                                                                   means of a flexible tube (and camera) inserted into the tract,
               Inflammatory Bowel Disease (IBD) (colitis) IBD is a col-             usually from the anus, is gradually being replaced by the Pill-
               lective term for a number of inflammatory disorders. Ulcer-          Cam, a camera you swallow by mouth!
               ative colitis and Crohn’s disease are the most common of
               these. Ulcerative colitis affects the large intestine and rectum,
               but in Crohn’s disease, the inflammation can be more wide-
                                                                                       Check Your Progress 8.5
               spread and ulcers can penetrate more deeply. Ulcers are pain-        1. What are the different parts of the large intestine?
               ful and cause bleeding because they erode the submucosal             2. What are the functions of the large intestine?




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       156                Part II      Maintenance of the Human Body


       8.6 Nutrition and Weight Control                                                             How Obesity Is Defined
                                                                                                    Today, obesity is often defined as a body mass index (BMI)
       Obesity—that is, being grossly overweight, has doubled in
                                                                                                    of 35 or greater. The table in Figure 8.11 shows your BMI, as
       the United States in only 20 years. Nearly one-third (33%) of
                                                                                                    long as you already know your weight and height. The BMI
       adults are now obese. Furthermore, being overweight is now
                                                                                                    can also be calculated by taking the weight of the individual
       prevalent among children and adolescents. These statistics
                                                                                                    in kilograms and then dividing the weight by the square of
       are of great concern because excess body fat is associated
                                                                                                    the height in meters. Most people find that using the table is
       with a higher risk for premature death, diabetes type 2,
                                                                                                    a lot easier. As a general rule,
       hypertension, cardiovascular disease, stroke, gallbladder
       disease, respiratory disfunction, osteoarthritis, and certain                                            healthy BMIs=25.0 to 30.9;
       kinds of cancers.                                                                                        overweight BMIs=31.0 to 35.2;
           Obesity is also on the rise throughout the world. The                                                obese BMIs=35.3 to 39.9; and
       term globesity has been coined in recognition of the world-
                                                                                                                morbidly obese BMIs=40 or more.
       wide problem. In all regions, obesity appears to escalate as
       income increases. In Brazil and Colombia, the figure of over-                                      Your BMI is suppose to give you an idea of how much of
       weight people is about 40%—comparable with a number of                                       your weight is due to adipose tissue—that is, fat. In general,
       European countries.                                                                          the taller you are, the more you could weigh without it being
           The Health Focus on page 157 tells us about the various                                  due to fat. Using BMI in this way works for most people,
       unhealthy ways people have tried to keep their weight                                        especially if they tend to be sedentary. But your BMI number
       under control. The conclusion is that, while dieting, “Eat a                                 should be used as a general guide only because it does not
       variety of foods, watch your weight, and exercise.” To reverse a                             take into account fitness, bone structure, or gender. For exam-
       trend toward obesity, eat fewer calories, be more active, and                                ple, a weight lifter might have an obese BMI, not because he
       make wiser food choices.                                                                     has body fat, but because he is very muscular.



                                                                                       Body Mass Index (BMI)
                          18                 20              22                   24         26            28            30         32           34           36
                   6‘6”
                                    underweight
                                    healthy weight
                   6‘4”             overweight
                                    obese

                   6‘2”


                   6‘0”


                  5‘10”
         Height




                   5‘8”


                   5‘6”


                   5‘4”


                   5‘2”


                   5‘0”


                  4‘10”
                          50                 75             100                  125       150             175          200        225          250          275
                                                                                           Weight (lbs.)
       Source: U.S. Department of Agriculture: Dietary Guidelines for Americans 2005

       Figure 8.11 Determining your BMI.
       Match your weight with your height, then determine your body mass index (BMI). Healthy BMIs = 25.0 to 30.9; overweight BMIs = 31.0 to 35.2; obese BMIs =
       35.3 to 39.9; morbidly obese BMIs = 40 or more.




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                    Health Focus
                                 Searching for the Magic Weight-Loss Bullet
                  While dieting, “Eat a variety of foods, watch your weight, and ex-
                  ercise” doesn’t sound very glamorous, and not only that, you
                  can’t sell it to the public and make a lot of money. No wonder
                  the public, which is always looking for the magic weight-loss
                  bullet, is offered so many solutions to being overweight, most
                  of which are not healthy. The solutions involve trendy diet
                  programs, new prescription medications, and even surgery.
                  The latter two options are for people who, like Monica, have
                  tried a low-calorie diet and, hopefully, regular physical activ-
                  ity but have been unsuccessful in losing weight. Prescription
                  medications should only be taken under a physician’s supervi-
                  sion, and, of course, surgeries are only done by physicians.

                  TRENDY DIET PROGRAMS: Various diets for the over-
                  weight have been around for many years, and here are some
                  recently touted:                                                     suits against the manufacturers of the prescription drug fen-
                                                                                       phen for causing heart problems and the belief that a drug
                       The Pritikin Diet This diet encourages the consumption
                                                                                       called Meridia elevates blood pressure, new drugs continue to
                           of large amounts of carbohydrates and fiber in the
                                                                                       enter clinical trials. A drug called L-Marc is now being tested
                           form of whole grains and vegetables. The diet is so
                                                                                       around the nation, and there is no lack of volunteers. Another
                           low-fat that the dieter may not be able to consume a
                                                                                       one called Acomplia, which is suppose to block pleasure
                           sufficient amount of “healthy” fats.
                                                                                       receptors in the brain, is in late-stage clinical trials. Another
                       The Atkin's Diet This diet is just the opposite of the
                                                                                       idea is a weight-loss nasal spray. The availability of so many
                           Pritikin Diet because it is a low-carbohydrate (carb)
                                                                                       possible prescription drugs make it seem likely that the magic
                           diet. It is based on the assumption that if we eat
                                                                                       bullet may not be synonymous with the magic pill.
                           more protein and fat, our bodies lose weight by
                           burning stored body fat. The Atkin’s diet is thought        SURGICAL PROCEDURES: For people who are morbidly
                           by many to be a serious threat to homeostasis               obese, surgery may be recommended. There are three common
                           because it puts a strain on the body to maintain the        types of weight-loss surgery; gastroplasty, gastric bypass,
                           blood glucose level, the breakdown of fat lowers            and gastric banding. Gastroplasty involves partitioning or
                           blood pH, and the excretion of nitrogen from                stapling a small section of the stomach to reduce total food
                           protein breakdown stresses the kidneys.                     intake. Gastric bypass surgery involves attaching the lower
                       The Zone Diet and the South Beach Diet As a reaction to         part of the small intestine to the stomach, so that most of
                           the Atkin's diet, these diets recommend only “healthy”      the food bypasses both the stomach and small intestine,
                           fats and permit low-sugar carbs. In other words, these      decreasing the surface area for absorption. Gastric banding
                           diets are bringing us back, once again, to “Eat a variety   is a new procedure in which stomach size is reduced using
                           of foods, maintain your weight, and exercise.”              a constricting band. About one-third to one-half of people
                       You may also have heard of the caveman diet. A profes-          who received obesity surgery lose significant amounts
                  sor at Colorado State University goes back in time, claiming to      of weight and keep this weight off for at least five years.
                  have found the lifestyle change necessary for weight manage-         The downside of these operations is that people face the
                  ment. If you would rather just flush your system of toxins and        possibility of a lifetime of post-surgery problems, including
                  keep to your own lifestyle, some nutritionists claim that by         chronic diarrhea, vomiting, intolerance to dairy products,
                  consuming certain foods, such as cayenne pepper, mustard,            dehydration, and nutritional deficiencies, due to alterations
                  cinnamon, green vegetables, and omega-3-rich fish, you can            in nutrient digestion and absorption. Careful consideration
                  boost your metabolic rate and flush fat away.                         of the potential benefits and risks must be weighed.
                       Then again, according to a professor at Brigham Young                Many people also try liposuction to remove fat cells from
                  University, the cure for endless dieting and the key to reach-       localized areas in the body, but this is not recommended to
                  ing a healthy weight is to listen to your body. Using a hunger       treat obesity or morbid obesity. This procedure is clearly not
                  scale of 1 to 10, with 1 being starving and 10 being very over-      the solution to long-term weight loss, as the millions of fat
                  full, keep around 3 to 5 and you will eat less. Unfortunately,       cells that remain in the body after liposuction enlarge if the
                  such sensible advice doesn’t seem to have caught on yet.             person continues to overeat.
                                                                                            We are facing an obesity epidemic and must do something
                  PRESCRIPTION DRUGS: Ever since Health and Human                      about it. Are these weight-loss options the answer? There are
                  Services Secretary Tommy Thompson said that Medicare and             signs that the public is no longer searching for the magic bullet
                  Medicaid will consider obesity a disease, drugmakers have            and, instead, is focusing on “To achieve or maintain a healthy
                  revved up the search for a magic weight-loss pill. Despite law-      weight, eat a variety of healthy foods and exercise.”




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       158        Part II        Maintenance of the Human Body


       Classes of Nutrients                                                             Table 8.3           Reducing High-Glycemic Index Carbohydrates
       A nutrient can be defined as a component of food that per-
       forms a physiological function in the body. Nutrients pro-                       To reduce dietary sugar:
       vide us with energy, promote growth and development, and
       regulate cellular metabolism.                                                    1. Eat fewer sweets, such as candy, soft drinks, ice cream, and pastry.
                                                                                        2. Eat fresh fruits or fruits canned without heavy syrup.
       Carbohydrates
                                                                                        3. Use less sugar—white, brown, or raw—and less honey and syrups.
       Carbohydrates are either simple or complex. Glucose is a
                                                                                        4. Avoid sweetened breakfast cereals.
       simple sugar preferred by the body as an energy source.
       Complex carbohydrates with several sugar units are digested                      5. Eat less jelly, jam, and preserves.
       to glucose. While body cells can utilize fatty acids as an                       6. Eat fresh fruit; especially avoid artificial fruit juices.
       energy source, brain cells require glucose. For this reason                      7. When cooking, use spices, such as cinnamon, instead of sugar to flavor
       alone, it is necessary to include carbohydrates in the diet                         foods.
       because the body is unable to convert fatty acids to glucose.                    8. Do not put sugar in tea or coffee.
           Any product made from refined grains, such as white                           9. Avoid potatoes and processed foods made from refined carbohydrates,
       bread, cake, and cookies, should be minimized in the diet.                          such as white bread, rice, and pasta.
       During refinement, fiber and also vitamins and minerals
       are removed from grains, so primarily starch remains. In
       contrast, sources of complex carbohydrates, such as beans,
       peas, nuts, fruits, and whole-grain products, are recom-                        gators tell us that a chronically high insulin level may lead
       mended as a good source of vitamins, minerals, and also                         to insulin resistance, diabetes type 2, and increased fat de-
       fiber (Fig. 8.12). Insoluble fiber adds bulk to fecal material                    position. Deposition of fat is associated with coronary heart
       and stimulates movements of the large intestine, prevent-                       disease, liver ailments, and several types of cancer.
       ing constipation. Soluble fiber combines with bile salts and                          Table 8.3 gives suggestions on how to reduce your intake
       cholesterol in the small intestine and prevents them from                       of dietary sugars.
       being absorbed.
       Can Carbohydrates Be Harmful? Some nutritionists hy-                                 Carbohydrates are the preferred energy source for the
       pothesize that the high intake of refined carbohydrates and                           body, but select complex carbohydrates in whole grains,
       fructose sweeteners processed from cornstarch may be re-                             beans, nuts, and fruits, contain fiber, vitamins, and minerals
       sponsible for obesity in the United States. In addition, these                       in addition to carbohydrate.
       foods are said to have a high glycemic index (GI), because
       the blood glucose response to these foods is high. When the
       blood glucose level rises rapidly, the pancreas produces an
       overload of insulin to bring the level under control. Investi-
                                                                                       Proteins
                                                                                       Dietary proteins are digested to amino acids, which cells use
                                                                                       to synthesize hundreds of cellular proteins. Of the 20 differ-
                                                                                       ent amino acids, eight are essential amino acids that must
                                                                                       be present in the diet. Children will not grow if their diets
                                                                                       lack the essential amino acids. Eggs, milk products, meat,
                                                                                       poultry, and most other foods derived from animals contain
                                                                                       all nine essential amino acids and are “complete” or “high-
                                                                                       quality” protein sources.
                                                                                            Legumes (beans and peas) (Fig. 8.13), other types of veg-
                                                                                       etables, seeds and nuts, and also grains supply us with
                                                                                       amino acids, but each of these alone is an incomplete protein
                                                                                       source, because of a deficiency in at least one of the essential
                                                                                       amino acids. Absence of one essential amino acid prevents
                                                                                       utilization of the other 19 amino acids. Therefore, vegetari-
                                                                                       ans are counseled to combine two or more incomplete types
                                                                                       of plant products to acquire all the essential amino acids.
                                                                                       Also, tofu, soymilk, and other foods made from processed
                                                                                       soybeans are complete protein sources. A balanced vegetar-
       Figure 8.12 Fiber-rich foods.                                                   ian diet is quite possible with a little planning.
       Plants provide a good source of carbohydrates. They also provide a good              Amino acids are not stored in the body, and a daily
       source of vitamins, minerals , and fiber when they are not processed (refined).   supply is needed. However, it does not take very much




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                                                                                                             Chapter 8         Digestive System and Nutrition            159


               protein to meet the daily requirement. Two servings of                       Lipids
               meat a day (one serving is equal in size to a deck of cards)                 Fats, oils, and cholesterol are lipids. Saturated fats, which
               is usually plenty.                                                           are solids at room temperature, usually have an animal ori-
               Can Proteins Be Harmful? While the body is harmed if                         gin. Two well-known exceptions are palm oil and coconut
               the amount of protein in the diet is severely limited, it is                 oil, which contain mostly saturated fats and come from the
               also likely harmed if the diet contains an overabundance                     plants mentioned (Fig. 8.14). Butter and meats, such as mar-
               of protein. The average American eats about twice as much                    bled red meats and bacon, contain saturated fats.
               protein as needed, and some people may be on a diet that                          Oils contain unsaturated fatty acids, which do not pro-
               encourages the intake of proteins instead of carbohydrates                   mote cardiovascular disease. Corn oil and safflower oil are
               as an energy source. Also, bodybuilders, weight lifters, and                 high in polyunsaturated fatty acids. Polyunsaturated oils
               other athletes may include amino acid or protein supple-                     are nutritionally essential because they are the only type of
               ments in the diet because they think these supplements                       fat that contains linoleic acid and linolenic acid, two fatty
               will increase muscle mass. However, excess amino acids                       acids the body cannot make. Since these fatty acids must be
               are not always converted into muscle tissue. The liver re-                   supplied by diet, they are called essential fatty acids.
               moves the nitrogen portion during deamination and uses it                         Olive oil and canola oil are well known to contain a larger
               to form urea, which is excreted in urine. The water needed                   percentage of monounsaturated fatty acids than other types of
               for excretion of urea can cause dehydration when a person                    cooking oils. Omega-3 fatty acids—with a double bond in the
               is exercising and losing water by sweating. High-protein                     third position—are believed to be especially protective against
               diets, especially those rich in animal proteins, can also in-                heart disease. Some cold-water fishes, such as salmon, sardines,
               crease calcium loss in urine. Excretion of calcium can lead                  and trout, are rich sources of omega-3, but they contain only
               to kidney stones.                                                            about half that of flaxseed oil, the best source from plants.
                    Certain types of meat, especially red meat, are known                   Can Lipids Be Harmful? Cardiovascular disease is often
               to be high in saturated fats, while other sources of protein,                due to arteries blocked by plaque, which contains saturated
               such as chicken, fish, and eggs, are more likely to be low in                 fats and cholesterol. Cholesterol is carried in the blood by
               saturated fats. As discussed next, an intake of saturated                    two types of lipoproteins: low-density lipoprotein (LDL)
               fats leads to cardiovascular disease.                                        and high-density lipoprotein (HDL). LDL is thought of as
                                                                                            “bad” because it carries cholesterol from the liver to the
                    Sufficient proteins are needed to supply the essential                   cells, while HDL is thought of as “good” because it carries
                    amino acids. Meat and dairy sources of protein may supply               cholesterol to the liver, which takes it up and converts it to
                    unwanted saturated fat, but vegetable sources do not.                   bile salts. Saturated fats tend to raise LDL cholesterol levels,
                                                                                            while unsaturated fats lower LDL cholesterol levels.




                                                                                             canola oil       6               62                    32


                                                                                             safflower oil     9    12                    79


                                                                                             olive oil         14                    77                  9


                                                                                             beef fat                    51                    44         5 14


                                                                                             butter                       62                     33       5       33


                                                                                             coconut oil                        82                    11 7

                                                                                                                    Numbers in Percent

                                                                                                             % saturated fat                   % polyunsaturated fat
               Figure 8.13 Beans as food.                                                                    % monounsaturated fat             % cholesterol
               Beans are a good source of complex carbohydrates and protein. But beans
               don’t supply all nine of the essential amino acids. To ensure a complete     Figure 8.14 Fat and oils.
               source of protein in the diet, beans should be eaten in combination with a   This illustration gives the percentages of saturated and unsaturated fatty acids
               grain, such as rice.                                                         in fats and oils. Which one do you judge to be the most healthy to use?




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       160        Part II        Maintenance of the Human Body


           Trans-fatty acids (trans fats) arise when unsaturated
       fatty acids are hydrogenated to produce a solid fat. Found         Table 8.4          Reducing Certain Lipids
       largely in commercially produced products, trans fats may
       reduce the function of the cell-membrane receptors that            To reduce saturated fats and trans fats in the diet:
       clear cholesterol from the bloodstream. Trans fats are found
       in commercially packaged goods, such as cookies and                1. Choose poultry, fish, or dry beans and peas as a protein source.
       crackers; commercially fried foods, such as french fries           2. Remove skin from poultry, and trim fat from red meats before cooking;
       from some fast-food chains; packaged snacks, such as                  place on a rack so that fat drains off.
       microwaved popcorn; as well as in vegetable shortening             3. Broil, boil, or bake rather than fry.
       and some margarines. Any packaged goods that contain               4. Limit your intake of butter, cream, trans fats, shortenings, and tropical oils
       partially hydrogenated vegetable oils or “shortening” most            (coconut and palm oils).*
       likely contain trans fats.                                         5. Use herbs and spices to season vegetables instead of butter, margarine, or
           Table 8.4 gives suggestions on how to reduce dietary              sauces. Use lemon juice instead of salad dressing.
       saturated fat and cholesterol. It is not a good idea to rely on    6. Drink skim milk instead of whole milk, and use skim milk in cooking and
       commercially produced low-fat foods. In some products,                baking.
       sugars have replaced the fat, and in others, protein is used.      To reduce dietary cholesterol:


             Unsaturated fats such as those in oils do not lead           1. Avoid cheese, egg yolks, liver, and certain shellfish (shrimp and lobster).
                                                                             Preferably, eat white fish and poultry.
             to cardiovascular disease and are preferred. Fats and
             oils contain many more calories per gram than do             2. Substitute egg whites for egg yolks in both cooking and eating.
             carbohydrates and protein.                                   3. Include soluble fiber in the diet. Oat bran, oatmeal, beans, corn, and fruits,
                                                                             such as apples, citrus fruits, and cranberries, are high in soluble fiber.



       Minerals
       Minerals are divided into major minerals and trace miner-         Calcium
       als. The body contains more than 5 grams of each major min-       Calcium (Ca2) is a major mineral needed for the con-
       eral and less than 5 grams of each trace mineral. Table 8.5       struction of bones and teeth and for nerve conduction and
       lists the selected minerals and gives their functions and food    muscle contraction. Many people take calcium supple-
       sources. The major minerals are constituents of cells and         ments to counteract osteoporosis, a degenerative bone
       body fluids and are structural components of tissues.              disease that afflicts an estimated one-fourth of older men
            The trace minerals are parts of larger molecules. For        and one-half of older women in the United States. Osteo-
       example, iron (Fe2) is present in hemoglobin, and iodine         porosis develops because bone-eating cells called osteo-
       (I) is a part of thyroxine and triiodothyronine, hormones        clasts are more active than bone-forming cells called
       produced by the thyroid gland. Zinc (Zn2), copper (Cu2),        osteoblasts. Therefore, the bones are porous, and they
       and manganese (Mn2) are present in enzymes that cata-            break easily because they lack sufficient calcium. Due to
       lyze a variety of reactions. As research continues, more and      recent studies that show consuming more calcium does
       more elements are added to the list of trace minerals con-        slow bone loss in elderly people, the guidelines have been
       sidered essential. During the past three decades, for exam-       revised. A calcium intake of 1,000 mg a day is recom-
       ple, very small amounts of selenium, molybdenum,                  mended for men and for women who are premenopausal,
       chromium, nickel, vanadium, silicon, and even arsenic             and 1,300 mg a day is recommended for postmenopausal
       have been found to be essential to good health. Table 8.5         women. To achieve this amount, supplemental calcium is
       lists the functions of various minerals and gives their food      most likely necessary.
       sources and signs of deficiency and toxicity.                           Vitamin D is an essential companion to calcium in pre-
            Occasionally, individuals do not receive enough iron         venting osteoporosis. Other vitamins may also be helpful;
       (especially women), calcium, magnesium, or zinc in their          for example, magnesium has been found to suppress the
       diets. Adult females need more iron in their diet than males      cycle that leads to bone loss. In addition to adequate cal-
       (18 mg compared with 10 mg) because they lose hemoglo-            cium and vitamin intake, exercise helps prevent osteoporo-
       bin each month during menstruation. Stress can bring on a         sis. Risk factors for osteoporosis include drinking more
       magnesium deficiency, and due to its high-fiber content, a          than nine cups of caffeinated coffee per day and smoking.
       vegetarian diet may make zinc less available to the body.         Medications are also available that slow bone loss while
       However, a varied and complete diet usually supplies              increasing skeletal mass. These are still being studied for
       enough of each type of mineral.                                   their effectiveness and possible side effects.




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                 Table 8.5        Minerals

                 Mineral           Functions                           Food Sources                                     Conditions with

                                                                                                      Too Little                             Too Much

                 MAJOR (MORE THAN 100 MG/DAY NEEDED)

                 Calcium           Strong bones and teeth, nerve       Dairy products,                Stunted growth in children,            Kidney stones; interferes
                 (Ca2)            conduction, muscle contraction      leafy green vegetables         low bone density in adults             with iron and zinc absorption

                 Phosphorus        Bone and soft tissue growth;        Meat, dairy products,          Weakness, confusion,                   Low blood and bone
                 (PO43)           part of phospholipids,              sunflower seeds, food           pain in bones and joints               calcium levels
                                   ATP, and nucleic acids              additives

                 Potassium         Nerve conduction,                   Many fruits and                Paralysis, irregular                   Vomiting, heart attack,
                 (K)              muscle contraction                  vegetables, bran               heartbeat, eventual death              death

                 Sulfur (S2)      Stabilizes protein shape,           Meat, dairy products,          Not likely                             In animals, depresses
                                   neutralizes toxic substances        legumes                                                               growth

                 Sodium (Na)      Nerve conduction,                   Table salt                     Lethargy, muscle cramps,               Edema, high blood
                                   pH and water balance                                               loss of appetite                       pressure

                 Chloride (Cl)    Water balance                       Table salt                     Not likely                             Vomiting, dehydration

                 Magnesium         Part of various enzymes             Whole grains,                  Muscle spasm, irregular                Diarrhea
                 (Mg2)            for nerve and muscle                leafy green vegetables         heartbeat, convulsions,
                                   contraction, protein synthesis                                     confusion, personality changes

                 TRACE (LESS THAN 20 MG/DAY NEEDED)

                 Zinc (Zn2)       Protein synthesis, wound healing,   Meats, legumes,                Delayed wound healing,                 Anemia, diarrhea,
                                   fetal development and growth,       whole grains                   night blindness, diarrhea,             vomiting, renal failure,
                                   immune function                                                    mental lethargy                        abnormal cholesterol levels

                 Iron (Fe2)       Hemoglobin synthesis                Whole grains, meats,           Anemia, physical                       Iron toxicity disease, organ
                                                                       prune juice                    and mental sluggishness                failure, eventual death

                 Copper (Cu2)     Hemoglobin synthesis                Meat, nuts, legumes            Anemia, stunted growth                 Damage to internal
                                                                                                      in children                            organs if not excreted

                 Iodine (I)       Thyroid hormone synthesis           Iodized table salt, seafood    Thyroid deficiency                      Depressed thyroid function, anxiety

                 Selenium          Part of antioxidant enzyme          Seafood, meats, eggs           Vascular collapse, possible            Hair and fingernail
                 (SeO42)                                                                             cancer development                     loss, discolored skin

                 Manganese         Part of enzymes                     Nuts, legumes, green           Weakness and confusion                 Confusion, coma, death
                 (Mn2)                                                vegetables



               Sodium                                                                                Table 8.6         Reducing Dietary Sodium
               Sodium plays a major role in regulating the body’s water
               balance, as does chloride (Cl). The recommended amount                               To reduce dietary sodium:
               of sodium intake per day is 500 mg, although the average
               American takes in 4,000–4,700 mg every day. In recent                                 1. Use spices instead of salt to flavor foods.
               years, this imbalance has caused concern because sodium                               2. Add little or no salt to foods at the table, and add only small amounts of
               in the form of salt intensifies hypertension (high blood                                    salt when you cook.
               pressure) if you already have it. About one-third of the                              3. Eat unsalted crackers, pretzels, potato chips, nuts, and popcorn.
               sodium we consume occurs naturally in foods; another                                  4. Avoid hot dogs, ham, bacon, luncheon meats, smoked salmon,
               third is added during commercial processing; and we add                                    sardines, and anchovies.
               the last third either during home cooking or at the table in                          5. Avoid processed cheese and canned or dehydrated soups.
               the form of table salt.
                                                                                                     6. Avoid brine-soaked foods, such as pickles or olives.
                    Clearly, it is possible to cut down on the amount of sodium
               in the diet. Table 8.6 gives recommendations for doing so.                            7. Read nutrition labels to avoid high-salt products.




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       162        Part II        Maintenance of the Human Body


       Vitamins                                                                                 Vitamins C, E, and A are believed to defend the body
                                                                                           against free radicals, and therefore, they are termed antioxi-
       Vitamins are organic compounds (other than carbohydrate,                            dants. These vitamins are especially abundant in fruits and
       fat, and protein) that the body uses for metabolic purposes                         vegetables. The food pyramid in Figure 8.16 suggests we eat
       but is unable to produce in adequate quantity. Many vita-                           2½ cups of vegetables and 2 cups of fruit a day. To achieve
       mins are portions of coenzymes, which are enzyme helpers.                           this goal, think in terms of salad greens, raw or cooked veg-
       For example, niacin is part of the coenzyme NAD, and ribo-                          etables, dried fruit, and fruit juice, in addition to apples and
       flavin is part of another dehydrogenase, FAD, discussed in                           oranges and other fresh fruits.
       Chapter 3. Coenzymes are needed in only small amounts                                    Dietary supplements may provide a potential safeguard
       because each can be used over and over again. Not all vita-                         against cancer and cardiovascular disease, but nutritionists
       mins are coenzymes; vitamin A, for example, is a precursor                          do not think people should take supplements instead of
       for the visual pigment that prevents night blindness. If vita-                      improving their intake of fruits and vegetables. There are
       mins are lacking in the diet, various symptoms develop (Fig.                        many beneficial compounds in fruits that cannot be obtained
       8.15). Altogether, there are 13 vitamins, which are divided                         from a vitamin pill. These compounds enhance one anoth-
       into those that are fat soluble (Table 8.7) and those that are                      er’s absorption or action and also perform independent bio-
       water soluble (Table 8.8).                                                          logical functions.
       Antioxidants                                                                        Vitamin D
       Over the past 20 years, numerous statistical studies have been                      Skin cells contain a precursor cholesterol molecule that is
       done to determine whether a diet rich in fruits and vegetables                      converted to vitamin D after UV exposure. Vitamin D
       can protect against cancer. Cellular metabolism generates free                      leaves the skin and is modified first in the kidneys and then
       radicals, unstable molecules that carry an extra electron. The                      in the liver until finally it becomes calcitriol. Calcitriol pro-
       most common free radicals in cells are superoxide (O2) and                         motes the absorption of calcium by the intestines. The lack
       hydroxide (OH). In order to stabilize themselves, free radi-                       of vitamin D leads to rickets in children (Fig. 8.15a). Rick-
       cals donate an electron to DNA, to proteins (including                              ets, characterized by bowing of the legs, is caused by defec-
       enzymes), or to lipids, which are found in plasma membranes.                        tive mineralization of the skeleton. Most milk today is
       Such donations most likely damage these cellular molecules                          fortified with vitamin D, which helps prevent the occur-
       and thereby may lead to disorders, perhaps even cancer.                             rence of rickets.




       a.                                                  b.                                                 c.

       Figure 8.15 Illnesses due to vitamin deficiency.
       a. Bowing of bones (rickets) due to vitamin D deficiency. b. Dermatitis (pellagra) of areas exposed to light due to niacin (vitamin B3) deficiency. c. Bleeding gums
       are another early sign of vitamin C deficiency.




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                                                                                                                    Chapter 8    Digestive System and Nutrition                163


                 Table 8.7          Fat-Soluble Vitamins

                 Vitamin                 Functions                                Food Sources                                          Conditions with

                                                                                                                        Too Little                   Too Much

                 Vitamin A               Antioxidant synthesized from beta-       Deep yellow/orange and leafy,         Night blindness,             Headache, dizziness,
                                         carotene; needed for healthy eyes,       dark green vegetables, fruits,        impaired growth              nausea, hair loss,
                                         skin, hair, and mucous membranes,        cheese, whole milk, butter,           of bones and teeth           abnormal development
                                         and for proper bone growth               eggs                                                               of fetus

                 Vitamin D               A group of steroids needed for           Milk fortified with vitamin D,         Rickets,                     Calcification of soft tissues,
                                         development and maintenance of           fish liver oil; also made in the       decalcification and           diarrhea, possible
                                         bones and teeth                          skin when exposed to sunlight         weakening of bones           renal damage

                 Vitamin E               Antioxidant that prevents oxidation      Leafy green vegetables, fruits,       Unknown                      Diarrhea, nausea,
                                         of vitamin A and polyunsaturated         vegetable oils, nuts, whole-                                       headaches, fatigue,
                                         fatty acids                              grain breads and cereals                                           muscle weakness

                 Vitamin K               Needed for synthesis of substances       Leafy green vegetables,               Easy bruising                Can interfere with
                                         active in clotting of blood              cabbage, cauliflower                   and bleeding                 anticoagulant medication



                 Table 8.8          Water-Soluble Vitamins

                 Vitamin                 Functions                                Food Sources                                          Conditions with

                                                                                                                        Too Little                   Too Much

                 Vitamin C               Antioxidant; needed for forming          Citrus fruits, leafy green            Scurvy, delayed wound        Gout, kidney stones,
                                         collagen; helps maintain capillaries,    vegetables, tomatoes,                 healing, infections          diarrhea, decreased
                                         bones, and teeth                         potatoes, cabbage                                                  copper

                 Thiamine                Part of coenzyme needed for cellular     Whole-grain cereals,                  Beriberi, muscular           Can interfere
                 (vitamin B1)            respiration; also promotes activity of   dried beans and peas,                 weakness, enlarged           with absorption
                                         the nervous system                       sunflower seeds, nuts                  heart                        of other vitamins

                 Riboflavin               Part of coenzymes, such as FAD;*         Nuts, dairy products, whole-          Dermatitis, blurred          Unknown
                 (vitamin B2)            aids cellular respiration, including     grain cereals, poultry,               vision, growth failure
                                         oxidation of protein and fat             leafy green vegetables

                 Niacin                  Part of coenzymes NAD;+ needed           Peanuts, poultry, whole-              Pellagra, diarrhea,          High blood sugar
                 (nicotinic              for cellular respiration, including      grain cereals, leafy green            mental disorders             and uric acid,
                 acid)                   oxidation of protein and fat             vegetables, beans                                                  vasodilation, etc.

                 Folacin                 Coenzyme needed for production of        Dark leafy green                      Megaloblastic                May mask B12
                 (folic acid)            hemoglobin and formation of DNA          vegetables, nuts, beans,              anemia, spina bifida          deficiency
                                                                                  whole-grain cereals

                 Vitamin B6              Coenzyme needed for synthesis            Whole-grain cereals, bananas,         Rarely, convulsions,         Insomnia,
                                         of hormones and hemoglobin; CNS          beans, poultry, nuts, leafy           vomiting, seborrhea,         neuropathy
                                         control                                  green vegetables                      muscular weakness

                 Pantothenic             Part of coenzyme A needed for            Nuts, beans, dark green               Rarely, loss of appetite,    Unknown
                 acid                    oxidation of carbohydrates and fats;     vegetables, poultry,                  mental depression,
                                         aids in the formation of hormones        fruits, milk                          numbness
                                         and certain neurotransmitters

                 Vitamin B12             Complex, cobalt-containing compound;     Dairy products, fish,                  Pernicious anemia            Unknown
                                         part of the coenzyme needed for          poultry, eggs,
                                         synthesis of nucleic acids and myelin    fortified cereals

                 Biotin                  Coenzyme needed for metabolism of        Generally in foods, especially        Skin rash, nausea,           Unknown
                                         amino acids and fatty acids              eggs                                  fatigue


                 *FAD  flavin adenine dinucleotide
                 +
                  NAD  nicotinamide adenine dinucleotide




mad86867_ch08_143-168.indd 163                                                                                                                                              12/1/06 2:38:16 PM
                                                               SECOND PASS

       164        Part II        Maintenance of the Human Body


       How to Plan Nutritious Meals
       Many serious disorders in Americans are linked to a diet that
       results in excess body fat. While genetics is a factor in being
       overweight, a person cannot become fat without taking in
       more food energy (Calories, Kcal) than is needed. A person
       needs calories (energy) for their basal metabolism, which is
       the number calories your body burns at rest to maintain nor-
       mal body functions, and a person needs calories for exercise.
       The less exercise, the less calories that are needed beyond the
       basal metabolic rate. So, the first step in planning a diet is to
       limit the number of calories to an amount that you will use up
       each day. Let’s say you do all the necessary calculations
       (which are beyond the scope of this book) and discover that,
       as a woman, the maximum number of calories you can take in
       each day is 2,000, and as a man, you can afford 2,500 calories                      GRAINS            VEGETABLES              FRUITS           MILK        MEAT & BEANS
       without the anxiety of gaining weight.                                             Eat 6 oz.          Eat 21/2 cups         Eat 2 cups       Consume 3      Eat 51/2 oz.
            Now, the new food pyramid developed by the U.S. Depart-                       every day           every day            every day        cups every     every day
       ment of Agriculture (USDA) can be used to help you decide                                                                                     day; ages
                                                                                                                                                    2–8, 2 cups
       how those calories should be distributed among the foods to
                                                                                        Source: U.S. Dept. of Agriculture
       eat (Fig. 8.16). The new pyramid emphasizes food that should
       be eaten often and omits foods that should not be eaten on a                     Figure 8.16 The 2005 USDA food guide pyramid.
       regular basis. Additionally, the USDA provides recommenda-                       This food guide pyramid can be helpful when planning nutritious meals
       tions concerning the minimum quantity of foods in each group                     and snacks.
       that should be eaten daily. In general ,we should:
         • eat a variety of foods. Foods from all food groups
           should be included in the diet.                                                  dry beans and peas can be used to meet protein needs.
         • eat more of these foods: fruits, vegetables, whole                               Choose lean meats such as poultry, fish high in omega-
           grains, and fat-free or low-fat milk products. Choose                            3 fatty acids, such as salmon, trout, and herring, in
           dark-green vegetables, orange vegetables, and dry                                moderate sized portions. Include oils that are rich in
           beans and peas over potatoes, corn, or green peas more                           monounsaturated and polyunsaturated fatty acids in
           often. When eating grains, choose whole grains such as                           the diet. Choose and prepare foods and beverages with
           brown rice, oatmeal, whole wheat bread. Choose fruit                             little added sugars and salt.
           as a snack or a toping for foods, instead of sugar.                            • be physically active every day. If weight loss is needed,
         • eat less of foods high in saturated or trans fats, added                         decrease calorie intake slowly, while maintaining
           sugars, cholesterol, salt, and alcohol. Remember that                            adequate nutrient intake and increasing physical activity.


         Table 8.9            Preparing Meals and Snacks Using the Food Guide Pyramid

         Breakfast                      Snack                  Lunch                        Snack                           Dinner                      Dessert

         ½ cup oat flakes cereal         Whole-wheat muffin      Toasted American             Apple                           3 oz. hamburger             ½ cup frozen vanilla
                                                               cheese sandwich                                              patty                       yogurt topped with
                                                               (whole-grain bread)                                                                      sliced strawberries

         8 oz. nonfat milk              2 Tbsp peanut butter   1 cup vegetable salad        4 graham                        Whole-wheat bun
                                        mixed with 1 tsp                                    crackers
                                        sunflower seeds

         ½ cup blueberries              8 oz. nonfat milk      1 Tbsp olive oil and         ½ cup baked
                                                               vinegar salad dressing       beans

         1 slice whole-grain toast                             4 oz. tomato juice                                           1 cup steamed
                                                                                                                            broccoli spears

         1 tsp soft maragarine                                                                                              1 tsp soft maragarine

         4 oz. orange juice




mad86867_ch08_143-168.indd 164                                                                                                                                                    12/1/06 2:38:20 PM
                                                                 SECOND PASS

                                                                                                              Chapter 8      Digestive System and Nutrition            165




              a. Anorexia nervosa                                        b. Bulimia nervosa                                     c. Muscle dysmorphia

               Figure 8.17 Eating disorders.
               a. People with anorexia nervosa have a mistaken body image and think they are fat, even though they are thin. b. Those with bulimia nervosa overeat and then
               purge their bodies of the food they have eaten. c. People with muscle dysmorphia think their muscles are underdeveloped. They spend hours at the gym and are
               preoccupied with diet as a way to gain muscle mass.



               Table 8.9 gives you a sample menu, developed by a nutrition-                     weights are often normal and they tend to conceal their bing-
               ist, so you can see what a good diet for one day would look                      ing and purging practices. Women are more likely than men
               like. Table 8.9 also gives you an idea of what a nutritionist                    to develop bulimia; an estimated 4% of young women suffer
               means by a “serving size.”                                                       from this condition.
                                                                                                    Other abnormal eating practices include binge-eating dis-
                                                                                                order and muscle dysmorphia. Many obese people suffer
               Eating Disorders                                                                 from binge-eating disorder, a condition characterized by epi-
               People with eating disorders are dissatisfied with their body                     sodes of overeating that are not followed by purging. Stress,
               image. Social, cultural, emotional, and biological factors all                   anxiety, anger, and depression can trigger food binges. A per-
               contribute to the development of an eating disorder. Serious                     son suffering from muscle dysmorphia (Fig. 8.17c) thinks his
               conditions such as obesity, anorexia nervosa, and bulimia                        or her body is underdeveloped. Body-building activities and
               nervosa can lead to malnutrition, disability, and death.                         a preoccupation with diet and body form accompany this
               Regardless of the eating disorder, early recognition and                         condition. Each day, the person may spend hours in the gym
               treatment are crucial.                                                           working out on muscle-strengthening equipment. Unlike
                    Anorexia nervosa is a severe psychological disorder                         anorexia nervosa and bulimia, muscle dysmorphia affects
               characterized by an irrational fear of getting fat that results                  more men than women.
               in the refusal to eat enough food to maintain a healthy body
               weight (Fig. 8.17a). A self-imposed starvation diet is often
               accompanied by occasional binge eating that is followed by                            Check Your Progress 8.6
               purging and extreme physical activity to avoid weight gain.                        1. a. What is the rationale behind using BMI to judge obesity?
               Binges usually include large amounts of high-calorie foods,                           b. What disorders are associated with obesity?
               and purging episodes involve self-induced vomiting and                             2. Why might carbohydrates and why might fats be the cause of the
               laxative abuse. About 90% of people suffering from anorexia                           obesity epidemic today?
               nervosa are young women; an estimated 1 in 200 teenage                             3. a. What type fats are dangerous to our health, and (b) what type
               girls is affected.                                                                    is protective for cardiovascular disease?
                    A person with bulimia nervosa binge-eats, and then                            4. Proteins provide the essential amino acids; why shouldn’t they be
               purges to avoid gaining weight (Fig. 8.17b). The binge-purge                          eaten as an energy source?
               cycle behavior can occur several times a day. People with                          5. In general, what does the new USDA food pyramid tell you?
               bulimia nervosa can be difficult to identify because their body                     6. Name and describe four eating disorders.




mad86867_ch08_143-168.indd 165                                                                                                                                      12/1/06 2:38:30 PM
                                                             SECOND PASS

       166        Part II        Maintenance of the Human Body


       Summarizing the Concepts                                                  8.5 The Large Intestine and Defecation
                                                                                    • The large intestine consists of the cecum, the colon
                                                                                      (including the ascending, transverse, and descending
          8.1 Overview of Digestion                                                   colon), and the rectum, which ends at the anus.
                 The organs of the digestive system are located within              • The large intestine absorbs water, salts, and some
                 the GI tract. The processes of digestion require ingestion,          vitamins; forms the feces; and carries out defecation.
                 digestion, movement, absorption, and elimination. All              • Disorders of the large intestine include diverticulosis,
                 parts of the tract have four layers, called the mucosa,              irritable bowel syndrome, inflammatory bowel disease,
                 submucosa, muscularis, and serosa.                                   polyps, and cancer.
          8.2 First Part of the Digestive Tract                                  8.6 Nutrition and Weight Control
             • In the mouth, teeth chew the                                             The nutrients released by the
               food, saliva contains salivary                                           digestive process should
               amylase for digesting starch,                                            provide us with adequate
               and the tongue forms a bolus for                                         energy, essential amino
               swallowing.                                                              acids and fatty acids, and
             • The air passage and food                                                 all necessary vitamins
               passages cross in the pharynx.                                           and minerals. Today,
               During swallowing, the air
                                                                                        obesity is on the
               passage is blocked off by the soft
               palate and epiglottis; food enters                                       increase, possibly GRAINS VEGETABLES FRUITS    MILK  MEAT & BEANS

               the esophagus, and peristalsis                                           because people
               begins. The esophagus moves                                              eat too much food and make improper choices of food.
               food to the stomach.                                                     Obesity is associated with many illnesses, including
                                                                                        diabetes type 2 and cardiovascular disease. The food guide
          8.3 The Stomach and Small                                                     pyramid shows foods to emphasize and foods to minimize
              Intestine                                                                 for good health.
             • The stomach expands and                                              •   Carbohydrates are necessary in the diet, but simple sugars
               stores food and also churns, mixing food with the acidic                 and refined starches cause a rapid release of insulin that
               gastric juices. This juice contains pepsin, an enzyme that               can lead to diabetes type 2.
               digests protein.
                                                                                    •   Proteins supply essential amino acids.
             • The duodenum of the small intestine receives bile from
                                                                                    •   Unsaturated fatty acids, particularly the omega-3 fatty
               the liver and pancreatic juice from the pancreas. Bile
                                                                                        acids, are protective against cardiovascular disease.
               emulsifies fat and readies it for digestion by lipase.
                                                                                    •   Saturated fatty acids lead to plaque, which occludes
             • The pancreas produces enzymes that digest starch
                                                                                        blood vessels.
               (pancreatic amylase), protein (trypsin), and fat (lipase). The
               intestinal enzymes finish the process of chemical digestion.          •   Vitamins and minerals are also required by the body in
                                                                                        certain amounts.
             • Small nutrient molecules are absorbed at the villi in the
               walls of the small intestine.

          8.4 Three Accessory Organs and Regulation                             Understanding Key Terms
              of Secretions
                 Three accessory organs of digestion send secretions to the     absorption 145                       epiglottis 147
                 duodenum via ducts. These organs are the pancreas, liver,      anorexia nervosa 165                 esophagus 147
                 and gallbladder.                                               anus 154                             essential amino acids 158
             •   The pancreas produces pancreatic juice, which contains         appendix 145                         essential fatty acids 159
                 digestive enzymes for carbohydrate, protein, and fat.          bile 150, 152                        fiber 154
             •   The liver produces bile, destroys old blood cells, detoxifies   binge-eating disorder 165            gallbladder 152
                 blood, stores iron, makes plasma proteins, stores glucose      bolus 147                            gallstone 152
                 as glycogen, breaks down glycogen to glucose, produces         body mass index (BMI)-156            gastric 149
                 urea, and helps regulate blood cholesterol levels.             bulimia nervosa 165                  gastric gland 149
             •   The gallbladder stores bile, which is produced by the liver.   cecum 154                            glottis 147
                 The secretions of digestive juices                             chyme 144                            glycemic index (GI) 158
                 are controlled by the nervous                                  cirrhosis 153                        hard palate 146
                 system and by hormones.                                        colon 154                            heartburn 147
             •   Gastrin produced by the lower part                             constipation 154                     hemorrhoid 155
                 of the stomach stimulates via the                              defecation 154                       hepatitis 153
                 bloodstream the upper part of the                              dental caries 146                    hormone 152
                 stomach to secrete pepsin.                                     diaphragm 147                        ingestion 144
             •   Secretin and CCK produced by                                   diarrhea 154                         jaundice 153
                 the duodenal wall stimulate the                                digestion 144                        lacteal 150
                 pancreas to secrete its juices and                             duodenum 150                         lactose intolerance 150
                 the gallbladder to release bile.                               elimination 145                      large intestine 154




mad86867_ch08_143-168.indd 166                                                                                                                          12/1/06 2:38:41 PM
                                                               SECOND PASS

                                                                                                   Chapter 8     Digestive System and Nutrition         167

               lipase 150                           pharynx 147                        9. Tracing the path of food in the following list (a–f), which step
               liver 150                            plaque 159                            is out of order first?
               lumen 145                            polyp 155                             a. mouth                   d. small intestine
               mineral 160                          rectum 154                            b. pharynx                  e. stomach
               movement 144                         rugae 149                             c. esophagus                f. large intestine
               mucosa 145                           salivary amylase 146
               muscle dysmorphia 165                salivary gland 146                10. The appendix connects to the
               muscularis 145                       serosa 145                            a. cecum.                 d. liver.
               nutrient 158                         small intestine 150                   b. small intestine.       e. All of these are correct.
               obesity 156                          soft palate 146                       c. esophagus.
               osteoporosis 160                     sphincter 147                     11. Which association is incorrect?
               pancreas 152                         stomach 148                           a. mouth—starch digestion
               pancreatic amylase 152               submuscosa 145                        b. esophagus—protein digestion
               pepsin 149                           trypsin 152                           c. small intestine—starch, lipid, protein digestion
               periodontitis 146                    vermiform appendix 154                d. stomach—food storage
               peristalsis 147                      villus 150                            e. liver—production of bile
               peritonitis 145                      vitamin 162
                                                                                      12. Why can a person not swallow food and talk at the same time?
                     Match the key terms to these definitions.                             a. In order to swallow, the epiglottis must close off the trachea.
                                                                                          b. The brain cannot control two activities at once.
                a.            Essential requirement in the diet, needed in small          c. In order to speak, air must come through the larynx to
                     amounts. Often a part of a coenzyme.                                    form sounds.
               b.                Fat-digesting enzyme secreted by the pancreas.           d. A swallowing reflex is only initiated when the
                                                                                             mouth is closed.
                c.             Lymphatic vessel in an intestinal villus; it aids in       e. Both a and c are correct.
                     the absorption of fats.
                                                                                      13. Which association is incorrect?
               d.             Muscular tube for moving swallowed food from the            a. pancreas—produces alkaline secretions and enzymes
                     pharynx to the stomach.                                              b. salivary glands—produce saliva and amylase
                e.            Organ attached to the liver that serves to store and        c. gallbladder—produces digestive enzymes
                     concentrate bile.                                                    d. liver—produces bile

                                                                                      14. Which of the following could be absorbed directly
               Testing Your Knowledge of the Concepts                                     without need of digestion?
                                                                                          a. glucose                d. protein
                                                                                          b. fat                     e. nucleic acid
                1. Argue that absorption is the most important of the five                 c. polysaccharides
                   processes of digestion over the other four processes.
                   (pages 144–45)                                                     15. Peristalsis occurs
                                                                                          a. from the mouth to the small intestine.
                2. List the main organs of the digestive tract, and state                 b. from the beginning of the esophagus to the anus.
                   the contribution of each to the digestive process.                     c. only in the stomach.
                   (pages 146–47)                                                         d. only in the small and large intestine.
                3. Discuss the absorption of the products of digestion into the           e. only in the esophagus and stomach.
                   lymphatic and cardiovascular systems. (page 150)
                                                                                      16. An organ is a structure made of two or more tissues
                4. Name the enzymes involved in the digestion of starch,                  performing a common function. Which of the four tissue
                   protein, and fat, and tell where these enzymes are active and          types are present in the wall of the digestive tract?
                   what they do. (page 151)                                               a. epithelium              d. muscle tissue
                                                                                          b. connective tissue       e. All of these are correct.
                5. Why are the pancreas, liver, and gallbladder considered
                                                                                          c. nervous tissue
                   accessory organs of digestion and not an organ of
                   digestion? (page 152)                                              17. Which association is incorrect?
                6. Name and state the functions of the hormones that assist               a. protein—trypsin         d. maltose—pepsin
                   the nervous system in regulating digestive secretions.                 b. fat—bile                e. starch—amylase
                   (page 153)                                                             c. fat—lipase

                7. What is the chief contribution of each of these in the body—       18. Most of the products of digestion are absorbed across the
                   carbohydrates, proteins, fats, fruits, and vegetables? (pages          a. squamous epithelium of the esophagus.
                   158–59, 162)                                                           b. striated walls of the trachea.
                                                                                          c. convoluted walls of the stomach.
                8. Which three eating disorders involve binge eating? How are             d. fingerlike villi of the small intestine.
                   these three disorders different from one another? (page 165)           e. smooth wall of the large intestine.




mad86867_ch08_143-168.indd 167                                                                                                                       12/1/06 2:38:41 PM
                                                                 SECOND PASS

       168        Part II        Maintenance of the Human Body


        19. Bile                                                               34. Two layers of smooth muscle
            a. is an important enzyme for the digestion of fats.
                                                                               In questions 35–40, match each statement to an answer in the key.
            b. cannot be stored.
                                                                               Answers are used more than once. Some may have more than one
            c. is made by the gallbladder.
                                                                               answer.
            d. emulsifies fat.
            e. All of these are correct.                                       Key:
                                                                                   a. gastrin                 d. All of these are correct.
        20. Which of the following is not a function of the liver in adults?       b. secretin                e. None of these is correct.
            a. produces bile           d. produces urea                            c. CCK
            b. detoxifies alcohol       e. makes red blood cells
            c. stores glucose                                                  35. Stimulates gallbladder to release bile
        21. The large intestine                                                36. Hormone carried in bloodstream
            a. digests all types of food.                                      37. Stimulates the stomach to digest protein
            b. is the longest part of the intestinal tract.
            c. absorbs water.                                                  38. Enzyme that digests food
            d. is connected to the stomach.                                    39. Secreted by duodenum
            e. is subject to hepatitis.
                                                                               40. Secreted by the stomach
       In questions 22–26, match each function to an organ in the key.
                                                                               In questions 41–45, match each statement to a vitamin or mineral
       Key:                                                                    in the key.
           a. mouth                         b. esophagus
           c. stomach                       d. small intestine                 Key:
           e. large intestine                                                      a. calcium                      d. iodine
                                                                                   b. vitamin K                    e. vitamin A
        22. Removes nondigestible remains                                          c. sodium
        23. Serves as a passageway                                             41. Needed to make thyroid hormone
        24. Stores food                                                        42. Needed for night vision
        25. Absorbs nutrients                                                  43. Needed for bones, teeth, and muscle contraction
        26. Receives food                                                      44. Needed for nerve conduction, pH, and water balance
        27. The amino acids that must be consumed in the diet are called       45. Needed for making clotting proteins
            essential. Nonessential amino acids
            a. can be produced by the body.
            b. are only needed occasionally.
                                                                               Thinking Critically About the Concepts
            c. are stored in the body until needed.
                                                                               The opening story discusses bariatric surgery, which reduces
            d. can be taken in by supplements.
                                                                               the size of the stomach and enables food to bypass a section of
        28. Which of the following are often organic portions of               the small intestine. The surgery is generally done when obese
            important coenzymes?                                               individuals have unsuccessfully tried numerous ways to lose
            a. minerals                c. protein                              weight and their health is compromised by their weight. There
            b. vitamins               d. carbohydrates                         are many risks associated with the surgery, but it helps a number
        29. The products of digestion are                                      of people lose a considerable amount of weight. After people
            a. large macromolecules needed by the body.                        undergo the surgery, there are several lifestyle changes they must
            b. enzymes needed to digest food.                                  make to avoid nutritional deficiencies and to compensate for the
            c. small nutrient molecules that can be absorbed.                  small size of their stomach.
            d. regulatory hormones of various kinds.                           1a. Why do some people who had bariatric surgery process
            e. the food we eat.                                                    their food in a blender (or have to chew thoroughly) before
       In questions 30–34, match each statement to a layer of the wall of          swallowing their food?
       the esophagus in the key. Answers are used more than once.               b. Why should people who had bariatric surgery drink liquids
                                                                                   between meals rather than with meals?
       Key:
           a. mucosa                        c. muscularis                       2. What risk is there to the esophagus after bariatric surgery?
           b. submucosa                     d. serosa
                                                                                3. The text on page 149 describes how William Beaumont was
        30. Loose connective tissue that contains lymph nodules                    able to observe the workings of the stomach through a fistula
                                                                                   in St. Martin’s stomach.
        31. Contains a layer of epithelium that lines the lumen
                                                                                a. What do you think Beaumont observed after he inserted a
        32. Very thin layer of squamous epithelium that secretes a fluid,
                                                                                   piece of meat into St. Martin’s stomach?
            keeping the organ moist
                                                                                b. What do you think Beaumont observed after he inserted a
        33. Contains digestive glands and mucus-secreting goblet cells             piece of bread into St. Martin’s stomach?




mad86867_ch08_143-168.indd 168                                                                                                                      12/1/06 2:38:42 PM

				
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