The Digestive System and Body Metabolism

The Digestive System and Body Metabolism PART I: ANATOMY AND PHYSIOLOGY OF THE DIGESTIVE SYSTEM 14.1 Anatomy of the Digestive System We can separate the organs of the digestive system into two main groups: those forming the alimentary (al0e˘-men9tar-e; aliment 5 nourish) canal and the accessory digestive organs (Figure 14.1). The alimentary canal performs the whole menu of 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. 14.1a Organs of the Alimentary Canal The alimentary canal, also called the gastrointestinal (GI) tract or gut, is a continuous, coiled, hollow muscular tube that winds through the ventral body cavity from mouth to anus. Its organs are the mouth, pharynx, esophagus, stomach, small intestine, and large intestine. The large intestine leads to the terminal opening, or anus. In a cadaver, the alimentary canal is approximately 9 m (about 30 feet) long, but in a living person, it is considerably shorter because of its muscle tone. Food material within this tube is technically outside the body, because it has contact only with cells lining the tract and the tube is open to the external environment at both ends. This relationship becomes clearer if you think of your alimentary canal as an elongated doughnut. Then it’s easy to see that if you stick your finger through a doughnut hole, your finger is not really in the doughnut. Each organ of the alimentary canal is described next (find the organs in Figure 14.1). Mouth Food enters the digestive tract through the mouth, ororal cavity, a mucous membrane–lined cavity (Figure 14.2). The lips, or labia, protect its anterior opening; the cheeks form its lateral walls; the hard palate forms its anterior roof; and the soft palate forms its posterior roof. The uvula (u9vu-lah) is a fleshy fingerlike projection of the soft palate, which dangles from the posterior edge of the soft palate. The space between the lips and cheeks externally and the teeth and gums internally is the vestibule. The area contained by the teeth is the oral cavity proper. The muscular tongue occupies the floor of the mouth. The tongue has several bony attachments— two of these are to the hyoid bone and the styloid processes of the skull. The lingual frenulum (ling9gwal fren9u-lum), a fold of mucous membrane, secures the tongue to the floor of the mouth and limits its posterior movements (see Figure 14.2a). Homeostatic Imbalance 14.1 Children born with an extremely short lingual frenulum are often referred to as “tongue-tied” because movement of the tongue is restricted, leading to distorted speech. This congenital condition can be corrected surgically by cutting the frenulum. 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 oropharynx, posterior to the oral cavity; and the laryngopharynx, 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 longitudinally; 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 (per0 ı˘-stal9sis). Esophagus The esophagus (e˘-sof 9ah-gus), or gullet, runs from the pharynx through the diaphragm to the stomach. About 25 cm (10 inches) long, it is essentially a passageway that conducts food (by peristalsis) to the stomach. 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 14.3, p. 486): 1. The mucosa is the innermost layer, a moist mucous membrane that lines the hollow cavity, or lumen, of the organ. It consists primarily 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 stratified squamous epithelium, the epithelium is mostly simple columnar. 2. The submucosa is found just beneath the mucosa. It is soft connective tissue containing blood vessels, nerve endings, mucosa-associated lymphoid tissue (MALT), and lymphatic vessels. 3. The muscularis externa is a muscle layer typically made up of an inner circular layer and an outer longitudinal layer of smooth muscle cells. 4. The serosa is the outermost layer of the wall. As half of a serous membrane pair, the visceral peritoneum (per0ı˘-to-ne9um) consists of a single layer of flat, serous fluid–producing cells. The visceral peritoneum is continuous with the slippery parietal peritoneum, which lines the abdominopelvic cavity by way of a membrane extension, the mesentery (mes9en-ter0e). A mesentery is formed when two layers of peritoneum are fused together. Routes for nerves, blood vessels, and lymphatic vessels are found between the layers. Mesenteries anchor digestive organs in place and store fat. (These relationships are illustrated in Figure 14.5 on p. 488.) Homeostatic Imbalance 14.2 When the peritoneum is infected, a condition called peritonitis (per0ı˘-toni9tis), 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 important intrinsic nerve plexuses—the submucosal nerve plexus and the myenteric (mi-en9ter-ik; “intestinal muscle”) nerve plexus. These networks of nerve fibers are actually part of the autonomic nervous system. They help regulate the mobility and secretory activity of GI tract organs. Stomach The C-shaped stomach (Figure 14.4) 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 position near the heart), surrounds the cardioesophageal (kar0de-o-e˘-sof 0ahje9al) sphincter, through which food enters the stomach from the esophagus. The fundus is the expanded part of the stomach lateral to the cardial region. The body is the midportion of the stomach; in the body, the convex lateral surface is the greater curvature, and its concave medial surface is the lesser curvature. As it narrows inferiorly, the body becomes the pyloric antrum and then the funnelshape pylorus (pi-lor9us), the terminal part of the stomach. The pylorus is continuous with the small intestine through the pyloric sphincter, or pyloric valve. 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 (roo9ge; ruga 5 wrinkle, fold). The lesser omentum (o-men9tum), 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 14.5). The 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 breakdown of proteins begins in the stomach. The mucosa of the stomach is a simple columnar 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 or digestive 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 gastric juice. For example, some stomach cells produce intrinsic factor, a substance needed for absorption of vitamin B12 in the small intestine. The chief cells produce inactive proteindigesting 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 5 gut), produce local hormones, such as gastrin, that are important in regulating the digestive activities of the stomach (see Table 14.1). Most digestive activity occurs in the pyloric region of the stomach. After food has been processed in the stomach, it is thick like heavy cream and is called chyme (kıˉm). The chyme enters the small intestine through the pyloric sphincter. 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 muscular tube extending from the pyloric sphincter to the large intestine (see Figure 14.1, p. 483). 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. 488). The large intestine encircles and frames it in the abdominal cavity. The small intestine has three subdivisions: the duodenum (doo0uh-de9num; “twelve finger widths long”), the jejunum (je˘-joo9num; “empty”), and the ileum (il9e-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 (il0e-o-se9kal) valve (look ahead to Figure 14.8, p. 493). 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”) controls the movement of chyme into the small intestine 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 14.6). The main pancreatic and bile ducts join at the duodenum to form the flasklike hepatopancreatic ampulla (he˘-pa˘″to-pan-kre-a˘9tik am-pu9lah), literally, the “liver-pancreatic enlargement.” From there, the bile and pancreatic juice travel through the duodenal papilla and enter the duodenum together. Nearly all nutrient absorption occurs in the small intestine. The small intestine is well suited for its function. Its wall has three structures that increase the absorptive surface area tremendously—villi, microvilli, and circular folds (Figure 14.7). Villi are fingerlike projections of the mucosa that give it a velvety appearance and feel, much like the soft nap of a towel. Within each villus is a rich capillary bed and a modified lymphatic capillary called a lacteal. The nutrients are absorbed through the mucosal cells into both the capillaries and the lacteal (indicated in Figure 14.13 on p. 499). Microvilli (mi0kro-vil9i) are tiny projections of the plasma membrane of the mucosa cells that give the cell surface a fuzzy appearance, sometimes referred to as the brush border. The plasma membranes bear enzymes (brush border enzymes) that complete the digestion of proteins and carbohydrates in the small intestine. Circular folds also called plicae circulares (pli9se ser-ku-la9res), are deep folds of both mucosa and submucosa layers.Unlike the rugae of the stomach, the circular folds do not disappear when food fills the small intestine. Instead, they form an internal “corkscrew slide” to increase surface area and force chyme to travel slowly through the small intestine so nutrients can be absorbed efficiently. All these structural modifications (villi, microvilli, circular folds), which increase the surface area, decrease in number toward the end of the small intestine. In contrast, local collections of lymphatic tissue (called Peyer’s patches) found in the submucosa increase in number toward the end of the small intestine. This reflects the fact that the remaining (undigested) food residue in the intestine contains huge numbers of bacteria, which must be prevented from entering the bloodstream if at all possible. 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). Its major functions are to dry out the indigestible food residue by absorbing water and to eliminate these residues from the body as feces (fe9sˉez). It frames the small intestine on three sides and has these subdivisions: cecum (se9kum), appendix, colon, rectum, and anal canal. The saclike cecum is the first part of the large intestine. Hanging from the cecum is the wormlike appendix, a potential trouble spot. Because it can easily become obstructed, 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 (sig9moyd) colon. The sigmoid colon, rectum, and anal canal lie in the pelvis. The anal canal ends at the anus (a9nus), which opens to the exterior. The anal canal has two valves: the external anal sphincter, composed of skeletal muscle, is voluntary, and the internal anal sphincter, formed by smooth muscle, is involuntary. These sphincters, which act rather like purse strings to open and close the anus, are ordinarily closed except during defecation, 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 tremendous 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 of muscle called teniae coli (ten9ne-e ko9li; “ribbons of the colon”). Because these muscle bands usually display some degree of tone (are partially contracted), they cause the wall to pucker into small pocketlike sacs called haustra (haws9trah).