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463
PART I: ANATOMY AND
PHYSIOLOGY OF THE
DIGESTIVE SYSTEM
Anatomy of the Digestive System
➔ Learning Objectives
� Name the organs of the alimentary canal and accessory digestive organs, and identify each on an aRRroRriate diagram or model.
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hildren are fascinated by the workings of the digestive system: They relish crunching a potato chip, delight in making “mustaches” The Digestive System and $ody Metabolism
WHAT
HOW
WHY
The digestive system breaks down the food you eat into nutrients needed for metabolic processes, such as making ATP, and rids the body of materials that cannot be used, such as fiber. Chewing breaks down food into small pieces easy for enzymes to access; enzymes then chemically digest food into nutrients that are actively transported into blood and delivered to cells around the body. The digestive system is essential for providing the body with the energy and building blocks it requires to maintain life. with milk, and giggle when their stomach “growls.” As adults, we know that a healthy digestive system is essential for good health because it converts food into the raw materials that build and fuel our body’s cells. Specifically, the digestive system ingests food (takes it in), digests it (breaks it down) into nutrient molecules, absorbs the nutrients into the blood- stream, and then defecates (excretes) to rid the body of the indigestible wastes. INSTRUCTORS New $uilding Vocabulary Coaching Activities for this chapter are assignable in
464 Essentials of Human Anatomy and Physiology digestive functions (ingests, digests, absorbs, and defecates) as it propels the foodstuffs along its length. The accessory organs (teeth, tongue, and several large digestive glands) assist digestion in various ways, as described shortly. Organs of the Alimentary Canal The alimentary canal, also called the gastroin- testinal (GI) tract or gut, is a continuous, coiled, � Identify the overall function of the digestive system as digestion and absorRtion of foodstuffs, and describe the general activities of each digestive system organ. We can separate the organs of the digestive system into two main groups: those forming the alimen- tary (al″˘e-men′tar-e; aliment = nourish) canal and the accessory digestive organs (Figure .. The alimentary canal performs the whole menu of Mouth (oral cavity) Tongue Esophagus Liver Gallbladder Anus Duodenum Jejunum Ileum Small intestine Parotid gland Sublingual gland Submandibular gland Salivary glands Pharynx Stomach Pancreas (Spleen) Transverse colon Descending colon Ascending colon Cecum Sigmoid colon Rectum Appendix Anal canal Large intestine Figure . 6he human digestive system Alimentary canal and accessory organs. The organs of the alimentary canal have boldfacedlabels, whereas accessory organs do not. The liver and gallbladder are reflected superiorly and to the right side of the body.
466 Essentials of Human Anatomy and Physiology The walls of the alimentary canal organs from! the esophagus to the large intestine are made!up of the same four tissue layers, or tunics (Figure . :
- The mucosa is the innermost layer, a moist mucous membrane that lines the hollow cav- ity, or lumen, of the organ. It consists primar- ily of surface epithelium plus a small amount of connective tissue (lamina propria) and a scanty smooth muscle layer. Beyond the esophagus, which has a friction-resisting strati- fied squamous epithelium, the epithelium is mostly simple columnar.
- The submucosa is found just beneath the mucosa. It is soft connective tissue contain- ing blood vessels, nerve endings, mucosa- associated lymphoid tissue (MALT), and lymphatic vessels.
- The muscularis externa is a muscle layer typically made up of an inner circular layer and an outer longitudinal layer of smooth muscle cells.
- The serosa is the outermost layer of the wall. As half of a serous membrane pair, the vis- ceral peritoneum (per″˘ı-to-ne′um) consists of a single layer of flat, serous fluid–producing cells. The visceral peritoneum is continu- ous with the slippery parietal peritoneum, which lines the abdominopelvic cavity by way of a membrane extension, the mesentery (mes′en-ter″e). (These relationships are illus- trated in Figure 14.5 on p. 470.) *omeostatic Imbalance . When the peritoneum is infected, a condition called peritonitis (per″˘ı-to-ni′tis), the peritoneal membranes tend to stick together around the infection site. This helps to seal off and localize many intraperitoneal infections (at least initially), providing time for macrophages in the lymphatic tissue to mount an attack. ____________________✚ The alimentary canal wall contains two impor- tant intrinsic nerve plexuses—the submucosal nerve plexus and the myenteric (mi-en′ter-ik; “intestinal muscle”) nerve plexus. These net- works of nerve fibers are actually part of the auto- nomic nervous system. They help regulate the mobility and secretory activity of GI tract organs. condition can be corrected surgically by cutting the frenulum. _______________________________✚ At the posterior end of the oral cavity are paired masses of lymphatic tissue, the palatine tonsils. The lingual tonsil covers the base of the tongue just beyond. The tonsils, along with other lymphatic tissues, are part of the body’s defense system (look back at Figure 13.2, p. 439). When the tonsils become inflamed and enlarge, they par- tially block the entrance into the throat (pharynx), making swallowing difficult and painful, as described in Homeostatic Imbalance 13.3, p. 438. As food enters the mouth, it is mixed with saliva and masticated (chewed). The cheeks and closed lips hold the food between the teeth during chewing. The nimble tongue continuously mixes food with saliva and initiates swallowing. Thus, the breakdown of food begins before it has even left the mouth. ➔
CONCEPTLINK
Recall that papillae containing taste buds, or taste receptors, are found on the tongue surface (see Chapter 8, p. 300). So, besides its food-manipulating function, the tongue allows us to enjoy and appreciate the food we eat. Pharynx From the mouth, food passes posteriorly into the oropharynx and laryngopharynx, both of which are common passageways for food, fluids, and air. The pharynx is subdivided into the nasopharynx, part of the respiratory passageway; the orophar- ynx, posterior to the oral cavity; and the laryngo- pharynx, which is continuous with the esophagus inferiorly (see Chapter 13). The walls of the pharynx contain two skeletal muscle layers. The cells of the outer layer run lon- gitudinally; those of the inner layer (the constrictor muscles) run around the wall in a circular fashion. Alternating contractions of these two muscle layers propel food through the pharynx inferiorly into the esophagus. Later we describe this propelling mechanism, called peristalsis (per″˘ı-stal′sis). 'soRhagus The esophagus (˘e-sof′ah-gus), or gullet, runs from the pharynx through the diaphragm to the stomach. About 25 cm (10 inches) long, it is essen- tially a passageway that conducts food (by peri- stalsis) to the stomach.
Chapter 14: The Digestive System and Body Metabolism 467
The stomach varies from 15 to 25 cm (6 to 10 inches) in length, but its diameter and volume depend on how much food it contains. When it is full, it can hold about 4 liters (1 gallon) of food. When it is empty, it collapses inward on itself, and its mucosa is thrown into large folds called rugae (roo′ge; ruga = wrinkle, fold). The lesser omentum (o-men′tum), a double layer of peritoneum, extends from the liver to the lesser curvature of the stomach. The greater omentum, another extension of the peritoneum, drapes downward and covers the abdominal organs like a lacy apron before attaching to the posterior body wall ( Figure .a , p. 470). The Stomach The C-shaped stomach ( Figure .a and b , p. 468) is on the left side of the abdominal cavity, nearly hidden by the liver and diaphragm. Different regions of the stomach have been named. The cardial region, or cardia (named for its posi- tion near the heart), surrounds the cardioesopha- geal (kar″de-o-˘e-sof″ah-je′al) sphincter, through which food enters the stomach from the esopha- gus. The fundus is the expanded part of the stom- ach lateral to the cardial region. The body is the midportion of the stomach; in the body, the con- vex lateral surface is the greater curvature, and its concave medial surface is the lesser curva- ture. As it narrows inferiorly, the body becomes the pyloric antrum and then the funnel-shaped pylorus (pi-lor′us), the terminal part of the stom- ach. The pylorus is continuous with the small intestine through the pyloric sphincter, or pyloric valve. Intrinsic nerve plexuses
- Myenteric nerve plexus
- Submucosal nerve plexus Submucosal glands Visceral peritoneum Mucosa
- Surface epithelium
- Lamina propria
- Muscle layer Submucosa Muscularis externa
- Longitudinal muscle layer
- Circular muscle layer Serosa (visceral peritoneum) Lumen Duct of gland Lymphoid tissue outside alimentary canal Gland in mucosa Nerve Artery Mesentery Vein Figure . $asic structure of the alimentary canal wall. ➔
CONCEPTLINK
Recall that valves control the flow of fluids, including blood flow in veins and through the heart (see Chapter 11, pp. 361–363). The valves of the digestive system (formed by sphincter muscles) control the flow of food and digestive juices through the GI tract.
Chapter 14: The Digestive System and Body Metabolism 469
The chief cells produce inactive protein-digesting enzymes, mostly pepsinogens. The parietal cells produce corrosive hydrochloric acid (HCl), which makes the stomach contents acidic and activates the enzymes, as in the conversion of pepsinogen to pepsin (shown in Figure 14.4d). The mucous neck cells produce a thin acidic mucus with an unknown function that is quite different from that secreted by the mucous cells of the mucosa. Still other cells, the enteroendocrine cells (entero = gut), produce local hormones, such as gastrin, that are important in regulating the digestive activities of the stomach (see 6able . on p. 471). Most digestive activity occurs in the pyloric region of the stomach. After food has been pro- cessed in the stomach, it is thick like heavy cream and is called chyme (kı"m). The chyme enters the small intestine through the pyloric sphincter.
&id ;ou )et It!
1. What is the sequential order (mouth to anus) of the digestive organs making up the alimentary canal? 2. In which organ of the alimentary canal does protein digestion begin? 3. The stomach epithelium secretes several substances, including alkaline mucus and intrinsic factor. What is the function of each of these two secretions? For answers, see ARRendiZ A. greater omentum is riddled with fat, which helps to insulate, cushion, and protect the abdominal organs. It also has large collections of lymphoid follicles containing macrophages and defensive cells of the immune system. The stomach acts as a temporary “storage tank” for food as well as a site for food breakdown. Besides the usual longitudinal and circular muscle layers, its wall contains a third, obliquely arranged layer in the muscularis externa (see Figure 14.4a). This arrangement allows the stomach not only to move food along the tract, but also to churn, mix, and pummel the food, physically breaking it down into smaller fragments. In addition, chemical break- down of proteins begins in the stomach. The mucosa of the stomach is a simple colum- nar epithelium composed entirely of mucous cells. They produce a protective layer of bicarbonate- rich alkaline mucus that clings to the stomach mucosa and protects the stomach wall from being damaged by acid and digested by enzymes. This otherwise smooth lining is dotted with millions of deep gastric pits, which lead into gastric glands (Figure 14.4c) that secrete the components of gas- tric juice. For example, some stomach cells pro- duce intrinsic factor, a substance needed for absorption of vitamin B 12 from the small intestine. Falciform ligament Liver Lesser omentum Gallbladder Stomach Duodenum Visceral peritoneum Transverse Greater omentum^ colon Small intestine Cecum Urinary bladder Diaphragm Pancreas Anus Rectum Peritoneal cavity Mesenteries Spleen Large intestine Parietal peritoneum Uterus (a) (b) Figure . Peritoneal attachments of the abdominal organs. (a Anterior view; the greater omentum is shown in its normal position, covering the abdominal viscera. (b Sagittal view of the abdominopelvic cavity of a female. Explore Cadaver
470 Essentials of Human Anatomy and Physiology widths long”), the jejunum (j˘e-joo′num; “empty”), and the ileum (il′e-um; “twisted intestine”), which contribute 5 percent, nearly 40 percent, and almost 60 percent of the length of the small intestine, respectively (see Figure 14.1). The ileum joins the large intestine at the ileocecal (il″e-o-se′kal) valve (look forward to Figure 14.8, p. 473). Chemical digestion of foods begins in earnest in the small intestine. The small intestine is able to process only a small amount of food at one time. The pyloric sphincter (literally, “gatekeeper”) con- trols the movement of chyme into the small intes- tine from the stomach and prevents the small intestine from being overwhelmed. In the C-shaped duodenum, some enzymes are produced by the intestinal cells. More important are enzymes that are produced by the pancreas and then delivered to the duodenum through the pancreatic ducts, where they complete the chemical breakdown of foods in the small intestine. Bile (formed by the liver) also enters the duodenum through the bile duct in the same area (Figure .. The main pan- creatic and bile ducts join at the duodenum to Small Intestine
➔ Learning Objective
� 'ZRlain how villi aid digestive Rrocesses in the small intestine. The small intestine is the body’s major digestive organ. Within its twisted passageways, usable nutrients are finally prepared for their journey into the cells of the body. The small intestine is a mus- cular tube extending from the pyloric sphincter to the large intestine (see Figure 14.1, p. 464). It is the longest section of the alimentary tube, with an average length of 2 to 4 m (7 to 13 feet) in a living person. Except for the initial part of the small intestine, which mostly lies in a retroperitoneal position (posterior to the parietal peritoneum), the small intestine hangs in sausagelike coils in the abdominal cavity, suspended from the posterior abdominal wall by the fan-shaped mesentery (Figure 14.5b, p. 470). The large intestine encircles and frames it in the abdominal cavity. The small intestine has three subdivisions: the duodenum (doo″ uh-de′num; “twelve finger Jejunum Cystic duct Duodenum Hepatopancreatic ampulla and sphincter Gallbladder Right and left hepatic ducts from liver Bile duct and sphincter Main pancreatic duct and sphincter Pancreas Common hepatic duct Duodenal papilla Accessory pancreatic duct Figure . 6he duodenum of the small intestine and related organs.
472 Essentials of Human Anatomy and Physiology Blood vessels serving the small intestine Muscle layers Villi (a) Small intestine (b) Villi (c) Absorptive cells Lumen Circular folds (plicae circulares) Absorptive cells Lacteal Intestinal crypt Lymphoid tissue Muscularis mucosae Submucosa Venule Lymphatic vessel Microvilli (brush border) Blood capillaries Villus They tremendously increase the surface area available for absorption of digested foodstuffs. A: Figure 14.7 Structural modifications of the small intestine. (a) Several circular folds (plicae circulares), seen on the inner surface of the small intestine. (b) Enlargement of a villus extension of the circular fold. (c) Enlargement of an absorptive cell to show microvilli (brush border). What is the functional value of the microvilli in the Q: absorptive cells of the small intestine? the intestine contains huge numbers of bacteria, which must be prevented from entering the bloodstream if at all possible.
Did You Get It?
4. Which muscular sphincter regulates the flow of chyme into the small intestine? 5. What are villi, and why are they important? For answers, see Appendix A. Large Intestine The large intestine is much larger in diameter than the small intestine (thus its name) but shorter in length. About 1.5 m (5 feet) long, it extends from the ileocecal valve to the anus ( Figure 14.8 , p. 473). Its major functions are to dry out the indi- gestible food residue by absorbing water and to eliminate these residues from the body as feces (fe′sez). It frames the small intestine on three sides! and has these subdivisions: cecum (se′kum), appendix, colon, rectum, and anal canal. The saclike cecum is the first part of the large intestine. Hanging from the cecum is the wormlike
Chapter 14: The Digestive System and Body Metabolism 473
two valves: the external anal sphincter, com- posed of skeletal muscle, is voluntary, and the internal anal sphincter, formed by smooth mus- cle, is involuntary. These sphincters, which act rather like purse strings to open and close the anus, are ordinarily closed except during defeca- tion, when feces are eliminated from the body. Because most nutrients have been absorbed before the large intestine is reached, no villi are present in the large intestine, but there are tremen- dous numbers of goblet cells in its mucosa that produce alkaline (bicarbonate-rich) mucus. The mucus lubricates the passage of feces to the end of the digestive tract. In the large intestine, the longitudinal layer of the muscularis externa is reduced to three bands appendix, a potential trouble spot. Because it is usually twisted, it is an ideal location for bacteria to accumulate and multiply. Inflammation of the appendix, appendicitis, is the usual result. The colon is divided into several distinct regions. The ascending colon travels up the right side of the abdominal cavity and makes a turn, the right colic (or hepatic) flexure, to travel across the abdominal cavity as the transverse colon. It then turns again at the left colic (or splenic) flexure and continues down the left side as the descending colon to enter the pelvis, where it becomes the S-shaped sigmoid (sig′moyd) colon. The sigmoid colon, rectum, and anal canal lie in the pelvis. The anal canal ends at the anus (a′nus), which opens to the exterior. The anal canal has Right colic (hepatic) flexure Transverse colon Haustrum Ascending colon IIeum (cut) IIeocecal valve Appendix Cecum Left colic (splenic) flexure Transverse mesocolon Descending colon Teniae coli Sigmoid colon Cut edge of mesentery External anal sphincter Rectum Anal canal Figure 14.8 The large intestine. A section of the cecum is removed to show the ileocecal valve.
Chapter 14: The Digestive System and Body Metabolism 475
the bony jaw. Dentin, a bonelike material, under- lies the enamel and forms the bulk of the tooth. It surrounds a central pulp cavity, which contains a number of structures (connective tissue, blood vessels, and nerve fibers) collectively called pulp. Pulp supplies nutrients to the tooth tissues and provides for tooth sensations. Where the pulp cav- ity extends into the root, it becomes the root canal, which provides a route for blood vessels, nerves, and other pulp structures to enter the pulp cavity of the tooth.
Did You Get It?
7. What is the general function of each of the four types of teeth? For the answer, see Appendix A. Salivary Glands Three pairs of salivary glands empty their secre- tions into the mouth. The large parotid (pah- rot′id) glands lie anterior to the ears. Mumps, a common childhood disease, is an inflammation of the parotid glands. If you look at the location of the parotid glands (see Figure 14.1), you can read- ily understand why people with mumps complain that it hurts to open their mouth or chew. The submandibular glands and the small sublingual (sub-ling′gwal) glands empty their secretions into the floor of the mouth through tiny ducts. The product of the salivary glands, saliva, is a mixture of mucus and serous fluids. The mucus moistens and helps to bind food together into a mass called a bolus (bo′lus), which makes chewing and swallowing easier. The clear serous portion contains an enzyme, salivary amylase (am′˘ı-las),! in a bicarbonate-rich (alkaline) juice that begins the process of starch digestion in the mouth. Saliva also contains substances such as lysozyme and antibod- ies (IgA) that inhibit bacteria; therefore, it has a pro- tective function as well. Last but not least, saliva dissolves food chemicals so they can be tasted. Pancreas The pancreas is a soft, pink, triangular gland that extends across the abdomen from the spleen to the duodenum (look back at Figure 14.6). Most of" the pancreas lies posterior to the parietal peritoneum; hence its location is referred to as ret- roperitoneal. Only the pancreas produces enzymes (described later) that break down all categories of digestible above the gingiva (jin′-j˘ıvah), or gum. The crown is covered with enamel, a ceramic-like substance as thick as a dime, that directly bears the force of chewing. It is the hardest substance in the body and is fairly brittle because it is heavily mineral- ized with calcium salts. The portion of the tooth embedded in the jaw- bone is the root; the root and crown are con- nected by a region called the neck. The outer surface of the root is covered by a substance called cement, which attaches the tooth to the periodontal (per″e-o-don′tal) membrane (liga- ment). This ligament holds the tooth in place in A:^ Dentin. Crown Neck Root Enamel Dentin Pulp cavity (contains blood vessels and nerves) Gum (gingiva) Cement Root canal Periodontal membrane (ligament) Bone What substance forms the bulk of the tooth? Q: Figure 14.10 Longitudinal section of a canine tooth.
476 Essentials of Human Anatomy and Physiology may occur when the gallbladder contracts (an event called a gallbladder attack). Blockage of the common hepatic or bile ducts (for example, by wedged gallstones) prevents bile from entering the small intestine, and it begins to accumulate and eventually backs up into the liver. This exerts pressure on the liver cells, and bile salts and bile pigments begin to enter the blood- stream. As the bile pigments circulate through the body, the tissues become yellow, or jaundiced. Blockage of the ducts is just one cause of jaun- dice. More often it results from actual liver prob- lems such as hepatitis, an inflammation of the liver, or cirrhosis (sir-ro′sis), a chronic inflamma- tory condition in which the liver is severely dam- aged and becomes hard and fibrous. Hepatitis is most often due to viral infection resulting from drinking contaminated water or transmitted in blood via transfusion or contaminated needles. Cirrhosis is almost guaranteed when someone drinks alcoholic beverages in excess for many years, and it is a common consequence of severe hepatitis. __________________________________ ✚
Did You Get It?
8. Mary has a dry mouth—very little saliva is being secreted. Digestion of which type of food will be affected (decreased) by this situation? 9. What is the digestive role of bile? What organ produces bile? 10. Which organ is the only one that produces enzymes capable of digesting all groups of food? For answers, see Appendix A.) Functions of the Digestive System
➔ Learning Objectives
� List and describe the six main activities of the digestive system. � Describe how foodstuffs in the digestive tract are mixed and moved along the tract. � Describe the function of local hormones in digestion. � List the major enzymes or enzyme groups involved in digestion, and name the foodstuffs on which they act. � Describe the mechanisms of swallowing, vomiting, and defecation. � Name the end products of protein, fat, and carbohydrate digestion. foods. The pancreatic enzymes are secreted into the duodenum in an alkaline fluid that neutralizes the"acidic chyme coming in from the stomach. The pancreas also has an endocrine function; it pro- duces the hormones insulin and glucagon (as explained in Chapter 9). Liver and Gallbladder The liver is the largest gland in the body. Located under the diaphragm, more to the right side of the body, it overlies and almost completely covers the stomach (look back at Figure 14.5a). The liver has four lobes and is suspended from the diaphragm and abdominal wall by a deli- cate" mesentery cord, the falciform (fal′s˘ı-form) ligament. The liver has many critical metabolic and regulatory roles; however, its digestive function is" to produce bile. Bile leaves the liver through the common hepatic duct and enters the duo- denum through the bile duct (look back at Figure"14.6). Bile is a yellow-to-green, watery solution con- taining bile salts, bile pigments (chiefly bilirubin, a breakdown product of hemoglobin), cholesterol, phospholipids, and a variety of electrolytes. Of these components, only the bile salts (derived from cholesterol) and phospholipids help the digestive process. Bile does not contain enzymes, but its bile salts emulsify fats by physically break- ing large fat globules into smaller ones, thus pro- viding more surface area for the fat-digesting enzymes to work on. The gallbladder is a small, thin-walled green sac that snuggles in a shallow fossa in the inferior surface of the liver (look back at Figure 14.6). When food digestion is not occurring, bile backs up the cystic duct and enters the gallbladder to be stored. While in the gallbladder, bile is con- centrated by the removal of water. Later, when fatty food enters the duodenum, a hormonal stimulus prompts the gallbladder to contract and spurt out stored bile, making it available to the duodenum. Homeostatic Imbalance 14. 4 If bile is stored in the gallbladder for too long or too much water is removed, the cholesterol it con- tains may crystallize, forming gallstones. Because gallstones tend to be quite sharp, agonizing pain
478 Essentials of Human Anatomy and Physiology when we talk about blood sugar level, we are referring to glucose. Fructose is the most abundant sugar in fruits, and galactose is found in milk. The only carbohydrates that our digestive sys- tem digests, or breaks down to simple sugars, are sucrose (table sugar), lactose (milk sugar), maltose (malt sugar), and starch. Sucrose, maltose, and lac- tose are referred to as disaccharides, or double sugars, because each consists of two simple sugars linked together. Starch is a polysaccharide (liter- ally, “many sugars”) formed by linking hundreds of glucose units. Although we eat foods containing other polysaccharides, such as cellulose, we do not have enzymes capable of breaking them down. The indigestible polysaccharides do not provide us with any nutrients, but they help move the foodstuffs along the gastrointestinal tract by pro- viding bulk, or fiber, in our diet. Proteins are digested to their building blocks, which are amino (ah-me′no) acids. The interme- diate product of protein digestion is polypeptides. When lipids (fats) are digested, they yield two different types of building blocks—fatty acids and an alcohol called glycerol (glis′er-ol). The diges- tion, or chemical breakdown, of carbohydrates, proteins, and fats is summarized in Figure 14.13 , and we describe it in more detail shortly.
- Absorption. Absorption is the transport of digestive end products from the lumen of the GI tract to the blood or lymph. For absorp- tion to occur, the digested foods must first enter the mucosal cells by active or passive transport processes. The small intestine is the major absorptive site.
- Defecation. Defecation is the elimination of indigestible residues from the GI tract via the anus in the form of feces. Some of these processes are the job of a sin- gle organ. For example, only the mouth ingests, and only the large intestine defecates. But most digestive system activities occur bit by bit as food is moved along the tract. Thus, in one sense, the digestive tract can be viewed as a “disassembly line” in which food is carried from one stage of its processing to the next and its nutrients are made available to the cells in the body along the way. Throughout this book we have stressed the body’s drive to maintain a constant internal envi- ronment, particularly in terms of homeostasis of the blood, which comes into contact with all body Because each of the major food groups has very different building blocks, let’s take a little time to review these organic molecules (which we first introduced in Chapter 2). The building blocks, or units, of carbohydrate foods are monosaccha- rides (mon″o-sak′ah-rı!dz), or simple sugars. We need to remember only three of these that are common in our diet—glucose, fructose, and galac- tose. Glucose is by far the most important, and (a) (b) Figure 14.12 Peristaltic and segmental move ments of the digestive tract. (a) In peristalsis, adjacent or neighboring segments of the intestine (or other alimentary canal organs) alternately contract and relax, thereby moving food distally along the tract. (b) In segmentation, single segments of the intestine alternately contract and relax. Because active segments are separated by inactive ones, the food is moved forward and then backward. Thus the food is mixed rather than simply propelled along the tract.
Starch and disaccharides Oligosaccharides* and disaccharides Lactose Maltose Sucrose Glucose Fructose Salivary amylase (^) Mouth Pancreatic amylase Brush border enzymes in small intestine (dextrinase, glucoamylase, lactase, maltase, and sucrase) Small intestine Small intestine Small intestine Small intestine Small intestine Small intestine Stomach Foodstuff Enzyme(s) and source Site of action Protein Large polypeptides Pepsin (stomach glands) in the presence of HCl 7nemulsiƂed fats EmulsiƂed Dy the detergent action of bile salts from the liver Pancreatic lipase Small polypeptides Pancreatic enzymes (trypsin, chymotrypsin, carboxypeptidase) Amino acids (some dipeptides and tripeptides) Monoglycerides and fatty acids *Oligosaccharides consist of a few linked monosaccharides. Glycerol and fatty acids Brush border enzymes (aminopeptidase, carboxypeptidase, and dipeptidase) Galactose The monosaccharides glucose, galactose, and fructose enter the capillary blood in the villi and are transported to the liver via the hepatic portal vein. Amino acids enter the capillary blood in the villi and are transported to the liver via the hepatic portal vein. Fatty acids and monoglycerides enter the lacteals of the villi and are transported to the systemic circulation via the lymph in the thoracic duct. (Glycerol and short-chain fatty acids are absorbed into the capillary blood in the villi and transported to the liver via the hepatic portal vein.) Digestion of carbohydrates Digestion of proteins Digestion of fats Absorption of proteins Absorption of carbohydrates Absorption of fats Figure 14.13 Flowchart of digestion and absorption of foodstuffs. 479
Chapter 14: The Digestive System and Body Metabolism 481
Activities of the Stomach Food Breakdown Secretion of gastric juice is regulated by both neural and hormonal factors. The sight, smell, and taste of food stimulate parasympathetic nervous system reflexes, which increase the secretion of gastric juice by the gastric (stomach) glands. In addition, the presence of food and a rising pH in the stomach stimulate the stomach cells to release the hormone gastrin. Gastrin prods the gastric glands to produce still more of the protein- digesting enzymes (such as pepsinogen), mucus, and hydrochloric acid. Under normal conditions, 2 to 3 liters of gastric juice are produced every day. Hydrochloric acid makes the stomach contents very acidic. This can be dangerous because both If we try to talk or laugh while swallowing, we confuse our bodies with mixed messages, and as a result food may enter the respiratory passages. This triggers still another protective reflex—cough- ing—during which air rushes upward from the lungs in an attempt to expel the food. Once food reaches the distal end of the esoph- agus, it presses against the cardioesophageal sphincter, causing it to open, and the food enters the stomach. The movement of food through the pharynx and esophagus is so automatic that a per- son can swallow and food will reach the stomach even if he is standing on his head. Gravity plays no part in the transport of food once it has left the mouth, which explains why astronauts in the zero gravity of outer space can still swallow and get nourishment. Tongue Trachea Pharynx Epiglottis up Glottis (lumen) of larynx Larynx up Bolus of food Upper esophageal sphincter contracted Upper esophageal sphincter relaxed Epiglottis down Esophagus Uvula Bolus Bolus Upper esophageal sphincter contracted Relaxed muscles Cardioesophageal sphincter open Cardioesophageal sphincter relaxed Upper esophageal sphincter 1 2 3 4 Figure 14.14 Swallowing. 1 2 4 5 3 6 With the upper esophageal sphincter contracted, the tongue pushes the food bolus posteriorly and against the soft palate. 1 2 4 5 3 6 The soft palate rises to close off the nasal passages as the bolus enters the pharynx. The larynx rises so that the epiglottis covers its opening as peristalsis carries the food through the pharynx and into the esophagus. The upper esophageal sphincter relaxes to allow food entry. 1 2 4 5 3 6 The upper esophageal sphincter contracts again as the larynx and epiglottis return to their former positions and peristalsis moves the food bolus inferiorly to the stomach. 1 2 4 5 3 6 The cardioesophageal sphincter opens, and food enters the stomach.
482 Essentials of Human Anatomy and Physiology on milk protein and converts it to a substance that looks like sour milk. Many parents mistakenly think that the curdy substance infants spit up after feeding is milk that has soured in their stomach. Rennin, pro- duced in large amounts in infants, is the same enzyme used to make milk curdle into cheese. It is not believed to be produced in adults. Other than the beginning of protein digestion, little chemical digestion occurs in the stomach. Except for aspirin and alcohol (which seem some- how to have a “special pass”), virtually no absorp- tion occurs through the stomach walls. As food enters and fills the stomach, its wall begins to stretch (at the same time the gastric juices are being secreted, as just described). Then the three muscle layers of the stomach wall become active. They compress and pummel the food, breaking it apart physically, all the while continuously mixing the food with the enzyme- containing gastric juice so that the thick fluid chyme is formed. The process looks something like the preparation of a cake mix, in which the floury mixture is repeatedly folded on itself and mixed with the liquid until it reaches a uniform texture. Food Propulsion Once the food has been well mixed, a rippling peristalsis begins in the upper half of the stom- ach ( Figure 14.15 , 1 ). The contractions increase in force as the food approaches the pyloric hydrochloric acid and the protein-digesting enzymes have the ability to digest the stomach itself, causing ulcers (see “A Closer Look” on p. 486). However, as long as enough mucus is made, the stomach is protected. Homeostatic Imbalance 14. 5 Occasionally, the cardioesophageal sphincter fails to close tightly and gastric juice backs up into the esophagus, which has little mucus protection. This results in a characteristic pain known as heart- burn, which, if uncorrected, leads to inflammation of the esophagus (esophagitis [˘e-sof′ah-ji′tis]) and perhaps even to ulceration of the esophagus. A common cause is a hiatal hernia, a structural abnormality in which the superior part of the stomach protrudes slightly above the diaphragm. Because the diaphragm no longer reinforces the relatively weak cardioesophageal sphincter, gastric juice flows into the unprotected esophagus. Conservative treatment involves restricting food intake after the evening meal, taking antacids, and sleeping with the head elevated. ______________✚ The extremely acidic environment that hydro- chloric acid provides is necessary, because it activates pepsinogen to pepsin, the active protein-digesting enzyme. Rennin, the second protein-digesting enzyme produced by the stomach, works primarily Propulsion: Peristaltic waves move from the fundus toward the pylorus. Grinding: The most vigorous peristalsis and mixing action occur close to the pylorus. The pyloric end of the stomach acts as a pump that delivers small amounts of chyme into the duodenum. Retropulsion: The peristaltic wave closes the pyloric valve, forcing most of the contents of the pylorus backward into the stomach. Pyloric valve closed Pyloric valve slightly opened Pyloric valve closed (^1 2 ) Figure 14.15 Peristaltic waves in the stomach.
