Understanding Normal and Clinical Nutrition Chapter 3 PDF

3 CHAPTER OUTLINE &...

3 CHAPTER OUTLINE & LEARNING OBJECTIVES 3.1 Digestion 69 Digestion, Absorption, LO 3.1 Explain how foods move through the digestive system, describing the actions of the organs, muscles, and digestive secretions along the way. and Transport 3.2 Absorption 76 LO 3.2 Describe the anatomical details of the intestinal cells that facilitate nutrient absorption. 3.3 The Circulatory Systems 79 LO 3.3 Explain how nutrients are routed in the circulatory systems from the GI tract into the body and identify which nutrients enter the blood directly and which must first enter the lymph. 3.4 The Health and Regulation of the GI Tract 82 LO 3.4 Describe how bacteria, hormones, and nerves influence the health and activities of the GI tract. Highlight 3 Common Digestive Problems 87 LO 3.5 Outline strategies to prevent or alleviate common GI problems. meaofoto/Shutterstock.com Copyright 2021 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.  Nutrition in Your Life Have you ever wondered what happens to the food you eat after you swallow it? Or how your body extracts nutrients from food? Have you ever marveled at how it all just seems to happen? Follow foods as they travel through the diges- tive system. Learn how a healthy digestive system takes whatever food you give it—whether sirloin steak and potatoes or tofu and brussels sprouts—and extracts the nutrients that will nourish the cells of your body. As you read this chapter, consider whether your current eating habits are supporting a healthy digestive system. Each cell in the body needs a continuous supply of many specific nutrients to maintain itself and carry out its work. These nutrients derive from the foods a person eats, but be- fore the body’s cells can use the nutrients, foods must first be broken down mechanically and chemically. This chapter follows the journey that breaks down foods into the nutrients featured in the later chapters. Then it follows the nutrients as they travel through the intestinal cells and into the body to do their work. As you read about the complexities and intricacies of these processes, take a mo- ment to appreciate the beauty and wisdom of the body. Recognize that the activities of the digestive system are finely coordinated and fully integrated with those of the cir- culatory, nervous, and hormonal systems. Then be thankful that your body can efficiently take care of its business without any direction from you, but know that it performs its best when you have given it optimal nourishment. This introduction presents a general overview of the processes common to all nutrients; later chapters discuss the specifics of digesting and absorbing individual nutrients. 3.1 Digestion LO 3.1 Explain how foods move through the digestive system, describing the actions of the organs, muscles, and digestive secretions along the way. Digestion is the body’s ingenious way of breaking down foods into nutrients in preparation for absorption (see Photo 3-1). In the process, the body overcomes many challenges without any conscious effort. Consider these challenges: 1. Human beings breathe, eat, and drink through their mouths. Air taken in through the mouth must go to the lungs; food and beverages must go to Hurst Photo/Shutterstock.com the stomach. The throat must be arranged so that swallowing and breathing don’t interfere with each other. 2. Positioned below the lungs is the diaphragm, a dome of muscle that sepa- rates the upper half of the torso from the lower half. The body needs a pas- sageway that will allow food from the mouth to pass through the diaphragm to reach the stomach below. > PHOTO 3-1 The process of digestion breaks 3. The contents of the digestive tract should be kept moving forward, slowly down all kinds of foods into nutrients. but steadily, at a pace that permits all reactions to reach completion. 4. To move through the system, food must be lubricated with fluids. Too much would form a liquid that would flow too rapidly; too little would form a paste digestive system: all the organs and glands associated with too dry and compact to move at all. The amount of fluids must be regulated to the ingestion and digestion of food. keep the intestinal contents at the right consistency to move along smoothly. digestion: the process by which food is broken down into absorbable units. 5. For digestive enzymes to work, foods must be broken down into small particles digest 5 take apart and suspended in enough liquid so that every particle is accessible. Once diges- absorption: the uptake of nutrients by the cells of the small tion is complete and nutrients have been absorbed from the GI tract into the intestine for transport into either the blood or the lymph. body, the remaining waste must be excreted. Excreting all the water along with absorb 5 suck in Digestion 69 Copyright 2021 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.  the solid residue, however, would be both wasteful and messy. Some water must be withdrawn, leaving a solid waste product that is easy to pass. 6. The digestive enzymes are designed to digest carbohydrate, fat, and protein. The cells of the GI tract are also made of carbohydrate, fat, and protein. These cells must be protected against the powerful digestive juices that they secrete. 7. Once waste matter has reached the end of the GI tract, it must be excreted, but it would be inconvenient and embarrassing if this function occurred continuously. Evacuation needs to occur periodically. The following sections show how the body elegantly and efficiently handles these challenges. Each section follows the GI tract from one end to the other—first de- scribing its anatomy, then its muscular actions, and finally its secretions. Anatomy of the Digestive Tract The gastrointestinal (GI) tract is a flex- ible muscular tube that extends from the mouth, through the esophagus, stom- ach, small intestine, large intestine, and rectum to the anus. Figure 3-1 traces the path followed by food from one end to the other. In a sense, the human body surrounds the GI tract. The inner space within the GI tract, called the lumen, is continuous from one end to the other. (GI anatomy terms appear in boldface type and are defined in Glossary 3-1.) A nutrient or other substance must pass through the GI tract’s wall to enter the body proper; some materials pass through the GI tract without being digested or absorbed. Mouth The process of digestion begins in the mouth. During chewing, teeth crush large pieces of food into smaller ones, and fluids from foods, beverages, and salivary glands blend with these pieces to ease swallowing.* Fluids also help dis- solve the food so that the tongue can taste it; only particles in solution can react with taste buds. When stimulated, the taste buds detect one, or a combination, of the five basic taste sensations: sweet, sour, bitter, salty, and umami (oo-MOM-ee), a savory flavor commonly associated with monosodium glutamate. In addition to these chemical triggers, aroma, appearance, texture, and temperature also influ- ence a person’s taste perceptions. gastrointestinal (GI) tract: the digestive tract. The principal organs are the stomach and intestines. The tongue provides taste sensations and moves food around the mouth, facili- gastro 5 stomach tating chewing and swallowing. When a mouthful of food is swallowed, it passes intestinalis 5 intestine through the pharynx, a short tube that is shared by the digestive system and GLOSSARY 3-1 esophageal (ee -SOFF-ah-GEE-al) sphincter: the circular muscle at the large intestine or colon (COAL-un): the lower portion of intestine that completes from the small intestine and regulates the flow of partially digested food into the small GI ANATOMY TERMS upper or lower end of the esophagus. the digestive process. Its segments are intestine; also called pylorus or pyloric valve. The lower esophageal sphincter is also the ascending colon, the transverse colon, pylorus 5 gatekeeper anus (AY-nus): the terminal outlet of the called the cardiac sphincter because of its the descending colon, and the sigmoid GI tract. proximity to the heart. colon. rectum: the muscular terminal part of appendix: a narrow blind sac the intestine, extending from the sigmoid esophagus (ee-SOFF-ah-gus): the food sigmoid 5 shaped like the colon to the anus. extending from the beginning of the pipe; the conduit from the mouth to the letter S (sigma in Greek) colon that contains bacteria and stomach. small intestine: a 10-foot length of lymph cells. lumen (LOO-men): the space within a small-diameter intestine that is the major gallbladder: the organ that stores and vessel such as the intestine. site of digestion of food and absorption of duodenum (doo-oh-DEEN-um or doo- concentrates bile. When it receives the ODD-num): the top portion of the small mouth: the oral cavity containing the nutrients. Its segments are the duodenum, signal that fat is present in the duodenum, tongue and teeth. jejunum, and ileum. intestine (about “12 fingers’ breadth” long the gallbladder contracts and squirts bile in ancient terminology). through the bile duct into the duodenum. pancreas: a gland that secretes sphincter (SFINK-ter): a circular muscle duodecim 5 twelve digestive enzymes and juices into the surrounding, and able to close, a body ileocecal (ill-ee-oh-SEEK-ul) valve: the duodenum. (The pancreas also secretes opening. Sphincters are found at epiglottis (epp-ih-GLOTT-iss): cartilage sphincter separating the small and large hormones into the blood that help specific points along the GI tract and in the throat that guards the entrance intestines. maintain glucose homeostasis.) regulate the flow of food particles. to the trachea and prevents fluid or ileum (ILL-ee-um): the last segment of pharynx (FAIR-inks): the passageway sphincter 5 band (binder) food from entering it when a person the small intestine. swallows. leading from the nose and mouth to the stomach: a muscular, elastic, saclike jejunum (je-JOON-um): the first larynx and esophagus, respectively. portion of the digestive tract that grinds epi 5 upon (over) two-fifths of the small intestine beyond pyloric (pie-LORE-ic) sphincter: the and churns swallowed food, mixing it with glottis 5 back of tongue the duodenum. circular muscle that separates the stomach acid and enzymes to form chyme. *The process of chewing is called mastication (mass-tih-KAY-shun). 70 Chapter 3 Digestion, Absorption, and Transport Copyright 2021 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. > FIGURE 3-1 The Gastrointestinal Tract Appendix INGESTION Houses bacteria and lymph cells Mouth (teeth and tongue) Small intestine Chews and mixes food Secretes enzymes that digest with saliva all energy-yielding nutrients to Salivary smaller nutrient particles; cells Pharynx glands of wall absorb nutrients into Directs food from mouth to blood and lymph esophagus Pharynx Epiglottis Mouth Ileocecal valve (sphincter) Salivary glands Allows passage from small to Secrete saliva Upper large intestine; prevents esophageal backflow from large intestine sphincter Trachea Epiglottis (to lungs) Protects airways during Pancreas swallowing Manufactures enzymes to digest all energy-yielding Trachea nutrients and releases Allows air to pass to and Esophagus bicarbonate to neutralize acid from lungs chyme that enters the small intestine Esophagus Lower Passes food from the mouth esophageal Pancreatic duct to the stomach sphincter Conducts pancreatic juice Diaphragm from the pancreas to the Esophageal sphincters small intestine Allow passage from mouth to esophagus and from Liver Stomach esophagus to stomach; Gallbladder prevent backflow from Pancreas stomach to esophagus and Pyloric from esophagus to mouth sphincter Pancreatic duct Diaphragm Separates the abdomen from Small intestine Bile (duodenum, the thoracic cavity duct jejunum, ileum) Stomach Ileocecal Churns, mixes, and grinds valve food to a liquid mass; adds Large intestine (colon) acid, enzymes, and fluid Absorbs water and minerals; Appendix passes waste (fiber, bacteria, Pyloric sphincter and unabsorbed nutrients) Allows passage from stomach along with water to the rectum to small intestine; prevents Large intestine backflow from small intestine (colon) Rectum Liver Stores waste prior to Manufactures bile salts, Rectum elimination detergent-like substances, to help digest fats Anus Anus Gallbladder Holds rectum closed; Stores bile until needed opens to allow elimination Bile duct Conducts bile from the gallbladder to the small ELIMINATION intestine the respiratory system. To bypass the entrance to the lungs, the epiglottis closes off the airway so that choking doesn’t occur when swallowing, thus resolving the first challenge. (Choking is discussed on pp. 87–88.) After a mouthful of food has been chewed and swallowed, it is called a bolus. bolus (BOH-lus): a portion; with respect to food, the amount Esophagus The esophagus has a sphincter muscle at each end. During a swal- swallowed at one time. low, the upper esophageal sphincter opens. The bolus then slides down the bolos 5 lump Digestion 71 Copyright 2021 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.  FIGURE 3-2 The Colon esophagus, which passes through a hole in the dia- The colon begins with the ascending colon rising upward toward the liver. It becomes phragm (challenge 2) to the stomach. The lower esoph- the transverse colon as it turns and crosses the body toward the spleen. The descending ageal sphincter at the entrance to the stomach closes colon turns downward and becomes the sigmoid colon, which extends to the rectum. behind the bolus so that it proceeds forward and doesn’t Along the way, the colon mixes the intestinal contents, absorbs water and salts, and slip back into the esophagus (challenge 3). forms stool. Stomach The stomach retains the bolus for a while in its upper portion before transferring it to the lower portion. As the stomach churns and adds digestive juices, the bolus becomes a semiliquid mass called chyme. Then, little by little, the stomach releases the chyme through Transverse the pyloric sphincter, which opens into the small intes- colon tine and then closes behind the chyme. Small Intestine At the beginning of the small intestine, the chyme bypasses the opening from the common bile duct, which is dripping f luids (challenge 4) into the Ascending small intestine from two organs outside the GI tract—the colon gallbladder and the pancreas. The chyme travels down End of small Opening from intestine the small intestine through its three segments—the duo- small intestine Descending denum, the jejunum, and the ileum—almost 10 feet of to large intestine colon Appendix tubing coiled within the abdomen. Rectum Large Intestine (Colon) Having traveled the length of Sigmoid the small intestine, the remaining contents arrive at an- Anus colon other sphincter (challenge 3 again): the ileocecal valve, located at the beginning of the large intestine (colon) in the lower right side of the abdomen. Upon entering the colon, the contents pass another opening. Should any intestinal contents slip into this opening, it would end up in the appendix, a blind sac about the size of your little finger. Normally, the contents bypass this opening, however, and travel along the large intestine up the right side of the abdomen, across the front to the left side, down to the lower left side, and finally below the other folds of the intestines to the back of the body, above the rectum (see Figure 3-2). As the intestinal contents pass to the rectum, the colon withdraws water, leav- ing semisolid waste (challenge 5). The strong muscles of the rectum and anal canal hold back this waste until it is time to defecate. Then the rectal muscles relax (challenge 7), and the two sphincters of the anus open to allow passage of the waste. The Muscular Action of Digestion In the mouth, chewing, the addition of saliva, and the action of the tongue transform food into a coarse mash that can be swallowed. After swallowing, all the activity that follows occurs without much conscious thought. As is the case with so much else that happens in the body, the muscles of the digestive tract meet internal needs without any concerted effort on your part. They keep things moving at just the right pace, slow enough to get chyme (KIME): the semiliquid mass of partly digested food the job done and fast enough to make progress. Movement of the digestive tract is expelled by the stomach into the duodenum. called GI motility. chymos 5 juice Segmentation and Peristalsis Two layers of muscles in the GI tract coordinate to GI motility (moh-TIL-ih-tee): the spontaneous movement of the GI tract that aids in the digestion and transit of the produce two kinds of action—segmentation and peristalsis. In segmentation, the contents within it. inner circular muscles contract and relax in a way that churns the chyme. This segmentation (SEG-men-TAY-shun): a periodic squeezing or churning action mixes the chyme with digestive juices and puts it in close contact partitioning of the intestine at intervals along its length by its with the absorbing cells of the intestinal wall. In peristalsis, the outer longitudinal circular muscles. muscles rhythmically contract in a way that moves chyme forward. These rhyth- peristalsis (per-ih-STALL-sis): wavelike muscular contractions of the GI tract that push its contents along. mic contractions occur continuously at varying rates and intensities, depending peri 5 around on the section of the GI tract and on whether food is present. Factors such as stellein 5 wrap stress, drugs, and illness may interfere with normal GI tract contractions. 72 Chapter 3 Digestion, Absorption, and Transport Copyright 2021 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. Stomach Action The stomach has the thickest walls and strongest muscles of > FIGURE 3-3 Stomach Muscles all the GI tract organs. In addition to the circular and longitudinal muscles, The stomach has three layers of muscles. it has a third layer of diagonal muscles that also alternately contract and relax (see Figure 3-3). These three sets of muscles work to force the chyme Longitudinal downward, but the pyloric sphincter (at the bottom of the stomach) usually remains tightly closed, preventing the chyme from passing into the duode- num of the small intestine. As a result, the chyme is churned and forced down, hits the pyloric sphincter, and remains in the stomach. Meanwhile, the stomach wall releases gastric juices. When the chyme is completely liq- Circular uefied with gastric juices, the pyloric sphincter opens briefly, about three times a minute, to allow small portions of chyme to pass through. At this Diagonal point, the chyme no longer resembles food in the least. Sphincter Contractions Sphincter muscles periodically open and close, allowing the contents of the GI tract to move along at a controlled pace (challenge 3 again). At the top of the esophagus, the upper esophageal sphincter opens in response to swallowing. At the bottom of the esopha- g us, the lower esophageal sphincter (sometimes called t he cardiac sphincter because of its proximity to the heart) prevents ref lux of the stom- ach contents. At the bottom of the stomach, the pyloric sphincter, which stays closed most of the time, holds the chyme in the stomach long enough for it to be thoroughly mixed with gastric juice and liquefied. The pyloric sphincter also prevents the intestinal contents from backing up into the stomach. At the end of the small intestine, the ileocecal valve performs a similar function, al- lowing the contents of the small intestine to empty into the large intestine. Finally, the tightness of the rectal muscle acts as a kind of safety device; to- gether with the two sphincters of the anus, it prevents continuous elimination (challenge 7). Figure 3-4 illustrates how sphincter muscles contract and relax to close and open passageways. The Secretions of Digestion The break- > FIGURE 3-4 An Example of a Sphincter Muscle down of food into nutrients requires secretions from five different organs: the salivary glands, When the circular muscles of a sphincter contract, the passage closes; when they relax, the passage opens. the stomach, the pancreas, the liver (via the gall- bladder), and the small intestine. These secre- Circular muscle Esophagus tions enter the GI tract at various points along the way, bringing an abundance of fluids (chal- Longitudinal muscle lenge 4) and a variety of enzymes. Esophagus muscles relax, Enzymes are formally introduced in Chap- opening the passageway. ter 6, but for now a simple definition will suf- fice. An enzyme is a protein that facilitates Diaphragm muscles relax, a c hemica l reac t ion—ma k ing a molec ule, Stomach opening the passageway. breaking a molecule apart, changing the ar- rangement of a molecule, or exchanging parts Esophagus muscles of molecules. As a catalyst, the enzyme itself contract, squeezing remains unchanged. The enzymes involved in on the inside. digestion facilitate a chemical reaction known as hydrolysis—the addition of water (hydro) Diaphragm muscles to break (lysis) a molecule into smaller pieces. contract, squeezing Glossary 3-2 (p. 74) describes how to identify on the outside. some of the common digestive enzymes and related terms; later chapters introduce specific enzymes. When learning about enzymes, it helps to know that the word ending reflux: a backward flow. -ase denotes an enzyme. Enzymes are often identified by the organ they come re 5 back from and the compounds they work on. Gastric lipase, for example, is a stomach flux 5 flow enzyme that acts on lipids, whereas pancreatic lipase comes from the pancreas catalyst (CAT-uh-list): a compound that facilitates chemical (and also works on lipids). reactions without itself being changed in the process. Digestion 73 Copyright 2021 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. > FIGURE 3-5 The Salivary Glands Saliva The salivary glands, shown in Figure 3-5, squirt just enough saliva to The salivary glands secrete enzyme-rich saliva into the moisten each mouthful of food so that it can pass easily down the esophagus mouth and begin the digestive process. Given (challenge 4). (Digestive glands and their secretions are defined in Glossary 3-3.) the short time food is in the mouth, salivary enzymes The saliva contains water, salts, mucus, and enzymes that initiate the digestion of contribute little to digestion. carbohydrates. Saliva also protects the teeth and the linings of the mouth, esopha- gus, and stomach from substances that might cause damage. Gastric Juice In the stomach, gastric glands secrete gastric juice, a mixture of water, enzymes, and hydrochloric acid, which acts primarily in protein diges- tion. The acid is so strong that it causes the sensation of heartburn if it happens to reflux into the esophagus. Highlight 3 discusses heartburn, ulcers, and other common digestive problems. The strong acidity of the stomach prevents bacterial growth and kills most bacteria that enter the body with food. It would destroy the cells of the stomach as well, but for their natural defenses. To protect themselves from gastric juice, the cells of the stomach wall (in fact, of the entire gastrointestinal lining) secrete Salivary mucus, a thick, slippery, white substance that coats the cells, protecting them glands from stomach acid, enzymes, and disease-causing bacteria that might otherwise cause harm (challenge 6). Figure 3-6 shows how the strength of acids is measured—in pH units. Note that the acidity of gastric juice registers below 2 on the pH scale—stronger than vinegar. The stomach enzymes work most efficiently in the stomach’s strong acid, but the salivary enzymes, which are swallowed with food, do not work in acid this strong. Consequently, the salivary digestion of carbohydrates ceases as stomach acid penetrates each newly swallowed bolus of food. Once in the stomach, sali- vary enzymes simply become proteins to be digested. Pancreatic Juice and Intestinal Enzymes By the time food leaves the stomach, digestion of all three energy nutrients (carbohydrates, fats, and proteins) has begun, and the action gains momentum in the small intestine. There the pan- creas contributes digestive juices by way of ducts leading into the duodenum. The pancreatic juice contains enzymes that act on all three energy nutrients, and the cells of the intestinal wall also possess digestive enzymes on their surfaces. In addition to enzymes, the pancreatic juice contains sodium bicarbonate, which is basic or alkaline—the opposite of the stomach’s acid (review Figure 3-6). The pancreatic juice thus neutralizes the acidic chyme arriving in the small in- testine from the stomach. From this point on, the chyme remains at a neutral or slightly alkaline pH. The enzymes of both the intestine and the pancreas work best in this environment. Bile Bile also flows into the duodenum. The liver continuously produces bile, which is then concentrated and stored in the gallbladder. The gallbladder squirts bile into the duodenum of the small intestine when fat arrives there. Bile is not an heartburn: a burning sensation in the chest area caused enzyme; it is an emulsifier that disperses fats in watery solutions, which gives the by backflow of stomach acid into the esophagus; medically digestive enzymes access to them. A summary of digestive secretions and their ac- known as gastroesophageal reflux. tions is presented in Table 3-1. pH: the unit of measure expressing a substance’s acidity or alkalinity. The lower the pH, the higher the H + ion concentration and the stronger the acid. A pH above 7 is The Final Stage By the time the contents of the GI tract reach the end of alkaline, or base (a solution in which OH – ions predominate). the small intestine, the three energy-yielding nutrients—carbohydrate, fat, and GLOSSARY 3-2 carbohydrase (KAR-boe-HIGH- drase), an enzyme that hydrolyzes digestive enzymes: proteins found in digestive juices that act on food to one and a hydroxyl group (OH) added to the other (from water, H 2O). (The noun is DIGESTIVE ENZYME TERMS carbohydrates. substances, causing them to break down hydrolysis; the verb is hydrolyze.) lipase (LYE-pase), an enzyme that into simpler compounds. hydro 5 water -ase (ACE): suffix denoting an enzyme. The root of the word often identifies hydrolyzes lipids (fats). hydrolysis (high-DROL-ih-sis): a chemical lysis 5 breaking the compound the enzyme works on. protease (PRO-tee-ase), an enzyme reaction in which one molecule is split into Examples include: that hydrolyzes proteins. two molecules, with hydrogen (H) added Copyright 2021 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.  > FIGURE 3-6 The pH of Common TABLE 3-1 Summary of Digestive Secretions and Their Major Actions Substances Organ or Gland Target Organ Secretion Action A substance’s acidity or alkalinity is measured in pH Salivary glands Mouth Saliva Fluid eases swallowing; salivary units. The pH is the negative logarithm of the hydrogen enzyme breaks down some ion concentration. Each increment represents a tenfold carbohydrate.* increase in concentration of hydrogen ions, meaning, Gastric glands Stomach Gastric juice Fluid mixes with bolus; hydrochloric for example, that a pH of 2 is 1000 times stronger than acid uncoils proteins; enzymes break a pH of 5. down proteins; mucus protects stomach cells.* Basic 14 Concentrated lye Pancreas Small intestine Pancreatic juice Bicarbonate neutralizes acidic gastric juices; pancreatic enzymes break down 13 Oven cleaner carbohydrates, fats, and proteins. 12 Liver Gallbladder Bile Bile is stored until needed. Gallbladder Small intestine Bile Bile emulsifies fat so that enzymes 11 Household ammonia can have access to break it down. Intestinal glands Small intestine Intestinal juice Intestinal enzymes break down 10 carbohydrate, fat, and protein fragments; mucus protects the 9 Baking soda intestinal wall. Bile *Saliva and gastric juice also contain lipases, but most fat breakdown occurs in the small intestine. 8 Pancreatic juice Blood pH neutral 7 Water protein—have been digested. Some vitamins and minerals are altered slightly dur- Saliva ing digestion, but most are absorbed as they are. All that remains—fluids, a few 6 Urine dissolved salts, and undigested materials such as fiber—enters the large intestine (colon). Fiber carries with it some fat, cholesterol, and a few minerals and retains 5 Coffee water as well, accounting for the consistency of stool. Intestinal bacteria ferment some fibers, producing water, gas, and small fragments of fat that provide energy 4 Orange juice for the cells of the colon. The colon itself retrieves the fluids and salts that the body 3 Vinegar can recycle. The waste that is finally excreted has little or nothing of value left in it. The body has extracted all that it can use from the food. Figure 3-7 (p. 76) sum- Lemon juice 2 Gastric juice marizes digestion by following a sandwich through the GI tract and into the body. 1 REVIEW IT Explain how foods move through the digestive system, describing the 0 Battery acid Acidic actions of the organs, muscles, and digestive secretions along the way. As Figure 3-1 (p. 71) shows, food enters the mouth and travels down the esophagus and through the upper and lower esophageal sphincters to the stomach, then through the pyloric sphincter to the small intestine, on through the ileocecal valve to the large intestine, past the appendix to the rectum, ending at the anus. The wavelike contractions of peristalsis and the periodic squeezing of segmentation keep things moving at a reasonable pace. Along the way, secretions from the salivary glands, stomach, pancreas, liver (via the gallbladder), and small intestine de- stool: waste matter discharged from the colon; also called liver fluids and digestive enzymes. feces (FEE-seez). GLOSSARY 3-3 portions that promotes the mixing of oils and fats in a watery solution. endo 5 inside krine 5 to separate pancreatic (pank-ree-AT-ic) juice: the exocrine secretion of the pancreas that DIGESTIVE GLAND contains both enzymes for the digestion gastric glands: exocrine glands in the AND SECRETION TERMS stomach wall that secrete gastric juice hydrochloric acid: an acid composed of carbohydrate, fat, and protein as well of hydrogen and chloride atoms (HCl) as bicarbonate, a neutralizing agent. The into the stomach. that is normally produced by the gastric bicarbonate: an alkaline compound with juice flows from the pancreas into the gastro 5 stomach glands. the formula HCO3 that is secreted from the small intestine through the pancreatic pancreas as part of the pancreatic juice. gastric juice: the digestive secretion of liver: the organ that manufactures bile, duct. (The pancreas also has an endocrine (Bicarbonate is also produced in all cell fluids the gastric glands of the stomach. among many other functions (described in function, the secretion of insulin and other from the dissociation of carbonic acid to help glands: cells or groups of cells that Chapter 7). hormones.) maintain the body’s acid-base balance.) secrete materials for special uses in the mucus (MYOO-kus): a slippery substance saliva: the secretion of the salivary bile: an emulsifier that prepares fats body. Glands may be exocrine (EKS-oh- secreted by cells of the GI lining (and glands. Its principal enzyme begins and oils for digestion; an exocrine crin) glands, secreting their materials other body linings) that protects the cells carbohydrate digestion. secretion made by the liver, stored in the “out” (into the digestive tract or onto the from exposure to digestive juices (and salivary glands: exocrine glands that gallbladder, and released into the small surface of the skin), or endocrine (EN-doe- other destructive agents). The lining of secrete saliva into the mouth. intestine when needed. crin) glands, secreting their materials “in” the GI tract with its coat of mucus is a emulsifier (ee-MUL-sih-fire): a substance (into the blood). mucous membrane. (The noun is mucus; with both water-soluble and fat-soluble exo 5 outside the adjective is mucous.) Copyright 2021 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. > FIGURE 3-7 The Digestive Fate of a Sandwich To review the digestive processes, follow a peanut butter and banana sandwich on whole-wheat, sesame seed bread through the GI tract. As the graph on the right illustrates, digestion of the energy nutrients begins in different parts of the GI tract, but all are ready for absorption by the time they reach the end of the small intestine. MOUTH: CHEWING AND SWALLOWING, WITH LITTLE DIGESTION Carbohydrate digestion begins as salivary amylase breaks down Carbohydrate starch from bread and peanut butter. Fiber covering on the sesame seeds is crushed by the teeth. Protein Fat digestion is minimal. Some hard fats melt as they reach body Fiber temperature. Fat Protein foods are moistened by saliva. STOMACH: COLLECTING AND CHURNING, WITH SOME DIGESTION Carbohydrate digestion ceases when the stomach acid of the gastric juices inactivates the salivary amylase. Proteins from the bread, seeds, and peanut butter begin to uncoil when they mix with the gastric acid, exposing them to the gastric protease enzymes that break down proteins. Fat from the peanut butter and seeds tends to separate from the watery GI juices. SMALL INTESTINE: DIGESTING AND ABSORBING Carbohydrate digestion picks up when the pancreas sends pancreatic enzymes to the small intestine via the pancreatic duct to break down starch. Enzymes on the surfaces of the small intestinal cells complete the process of breaking down starch into small fragments that can be absorbed through the cells of the small intestine walls and into the hepatic portal vein. Sugars from the banana require so little digestion that they begin to traverse the intestinal cells immediately on contact. Fat from the peanut butter and seeds is emulsified by bile. Now pancreatic and intestinal lipases can break down the fat to smaller fragments that can be absorbed through the cells of the small intestine wall and into the lymph. Protein breakdown depends on the pancreatic and intestinal proteases. Small fragments of protein are absorbed through the cells of the small intestine wall and into the hepatic portal vein. Vitamins and minerals are absorbed. Note: Sugars and starches are members of the carbohydrate family. ABSORPTION LARGE INTESTINE: ABSORBING AND ELIMINATING Fluids and some minerals are absorbed. Some fibers from the seeds, whole-wheat bread, peanut butter, and banana are partly digested by the bacteria living in the large intestine, and some of these products are absorbed. Most fibers pass through the large intestine and are excreted as feces; some fat, cholesterol, and minerals bind to fiber and are also excreted. E XC R E T I O N 3.2 Absorption LO 3.2 Describe the anatomical details of the intestinal cells that facilitate nutrient absorption. Within three or four hours after a person has eaten a dinner of beans and rice (or spinach lasagna, or steak and potatoes) with vegetable, salad, beverage, and dessert, the body must find a way to absorb the molecules derived from car- bohydrate, protein, and fat digestion—and the vitamin and mineral molecules 76 Chapter 3 Digestion, Absorption, and Transport Copyright 2021 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. as well. Most absorption takes place in the small intestine, one of the most el- egantly designed organ systems in the body. Within its 10-foot length, which provides a surface area equivalent to 1.5 bowling lanes, the small intestine traps and absorbs the nutrient molecules.1 To remove the absorbed molecules rapidly and provide room for more to be absorbed, a rush of circulating blood continu- Greg Amptman/Shutterstock.com ously washes the underside of this surface, carrying the absorbed nutrients away to the liver and other parts of the body. Figure 3-8 describes how most nutrients are absorbed by simple diffusion, facilitated diffusion, or active transport. Later chapters provide details on specific nutrients. Before following nutrients through the body, we must look more closely at the anatomy of the absorptive system. > PHOTO 3-2 If you have ever watched a sea Anatomy of the Absorptive System The inner surface of the small intes- anemone with its fingerlike projections in tine looks smooth and slippery, but when viewed through a microscope, it turns constant motion, you have a good picture of out to be wrinkled into hundreds of folds. Each fold is contoured into thousands how the intestinal villi move. of fingerlike projections, as numerous as the hairs on velvet fabric. These small in- testinal projections are called villi. A single villus, magnified still more, turns out to be composed of hundreds of cells, each covered with its own microscopic hairs, called microvilli (see Figure 3-9, p. 78). In the crevices between the villi lie the crypts—tubular glands that secrete the intestinal juices into the small intestine. Nearby goblet cells secrete mucus. The villi are in constant motion. Each villus is lined by a thin sheet of muscle, so it can wave, squirm, and wriggle like the tentacles of a sea anem- one (see Photo 3-2). Any nutrient molecule small enough to be absorbed is trapped among the microvilli and then drawn into the cells. Some partially digested nutrients are caught in the microvilli, digested further by enzymes there, and then absorbed into the cells. villi (VILL-ee or VILL-eye): fingerlike projections from the folds of the small intestine; singular villus. A Closer Look at the Intestinal Cells The cells of the villi are among the microvilli (MY-cro-VILL-ee or MY-cro-VILL-eye): tiny, hairlike projections on each cell of every villus that can trap nutrient most amazing in the body, for they recognize and select the nutrients the body particles and transport them into the cells; singular microvillus. needs and regulate their absorption. As already described, each cell of a villus is crypts (KRIPTS): tubular glands that lie between the intestinal coated with thousands of microvilli, which project from the cell’s membrane (re- villi and secrete intestinal juices into the small intestine. view Figure 3-9 and see Photo 3-3, p. 78). In these microvilli, and in the membrane, goblet cells: cells of the GI tract (and lungs) that secrete mucus. > FIGURE 3-8 Absorption of Nutrients Absorption of nutrients into intestinal cells typically occurs by simple diffusion, facilitated diffusion, or active transport. Occasionally, a large molecule is absorbed by endocytosis—a process in which the cell membrane engulfs the molecule, forming a sac that separates from the membrane and moves into the cell. Carrier loads Carrier loads Outside nutrient on nutrient on cell outside of cell... outside of cell... Cell En ergy membrane... and then... and then releases it on releases it on Inside inside of cell. inside of cell. cell SIMPLE FACILITATED ACTIVE DIFFUSION TRANSPORT TRANSPORT Some nutrients (such as water Some nutrients (such as the water-soluble Some nutrients (such as glucose and and small lipids) are absorbed vitamins) are absorbed by facilitated transport. amino acids) are absorbed actively. by simple diffusion. They cross They need a specific carrier to transport them These nutrients move against a into intestinal cells freely. from one side of the cell membrane to the concentration gradient, which requires other. (Alternatively, facilitated transport may energy. occur when the carrier changes the cell membrane in such a way that the nutrients can pass through.) Absorption 77 Copyright 2021 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.  > FIGURE 3-9 The Small Intestine Small intestine The walls of the small intestine are Each villus contains a network of Each villus is made of absorptive wrinkled into thousands of folds capillaries and lymphatic vessels for cells that are covered with even covered with villi. transporting nutrients out of the smaller projections—the intestinal cells. microvilli—that trap and absorb the nutrients. lie hundreds of different kinds of enzymes and “pumps,” which recognize and act on different nutrients. Descriptions of specific enzymes and pumps for each nutri- ent are presented in later chapters where appropriate; the point here is that the cells Microvilli are equipped to handle all kinds and combinations of foods and their nutrients. Specialized Cells The cells of the intestinal tract are specialized to absorb differ- ent nutrients. The nutrients that are ready for absorption early are absorbed near the top of the GI tract; those that take longer to be digested are absorbed farther down. Health care professionals who treat digestive disorders learn the special- Science Photo Library/Alamy Stock Photo