MED1002-Intestinal Phase of Digestion_.pdf
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Intestinal Phase of Digestion Learning Outcomes Describe the motility of the small intestine and colon. Explain the effects of hormones, paracrine, and neural pathways on digestion, secretions, and the motility of small intestine Explain the haustration, segmentation, propulsion, mass movement. Expl...
Intestinal Phase of Digestion Learning Outcomes Describe the motility of the small intestine and colon. Explain the effects of hormones, paracrine, and neural pathways on digestion, secretions, and the motility of small intestine Explain the haustration, segmentation, propulsion, mass movement. Explain the digestion process of carbohydrate, protein, and fat molecules in the small intestine Describe the roles of bacterial colonization of the colon in digestion and absorption of nutrients Explain the reflex responses in the GI tract. Explain the defecation mechanism. Small Intestine The small intestine is a tubular organ that extends from the pyloric sphincter to the beginning of the large intestine. With its many loops and coils, it fills much of the abdominal cavity. The small intestine receives secretions from the pancreas and liver. It also completes digestion of the nutrients in chyme, absorbs the products of digestion, and transports the residues to the large intestine. Hole's essentials of human anatomy and physiology, 11th edition Medical Physiology: A Cellular and Molecular Approach 2e The small intestine consists of three parts: the duodenum, the jejunum, and the ileum. The duodenum, about 25 centimeters long and 5 centimeters in diameter, lies posterior to the parietal peritoneum and is the most fixed portion of the small intestine. It follows a C-shaped path as it passes anterior to the right kidney and the upper three lumbar vertebrae. The remainder of the small intestine is mobile and lies free in the peritoneal cavity. The proximal twofifths of this portion of the small intestine is the jejunum, and the remainder is the ileum. A double-layered fold of peritoneal membrane called mesentery suspends these parts from the posterior abdominal wall Hole's essentials of human anatomy and physiology, 11th edition Once chyme passes into the small intestine, the intestinal phase of digestion begins. Chyme entering the small intestine has undergone relatively little chemical digestion, so its entry must be controlled to avoid overwhelming the small intestine. Motility in the small intestine is also controlled. Motility in Small Intestine Intestinal contents are slowly propelled forward by a combination of segmental and peristaltic (propulsion) contractions. Segmental contractions mix the luminal contents with pancreatic, biliary, and small-intestinal secretions, thus enhancing the digestion of dietary nutrients in the lumen. These segmental contractions also decrease the unstirred water layer that is adjacent to the apical membranes of the small intestine cells, thus promoting absorption. These movements occur following eating and are the result of contractions of circular muscle in segments flanked at either end by receiving segments that relax. However, it does not advance the luminal contents along the small intestine. Human Physiology an Integrated Approach, 8e Motility in Small Intestine Peristaltic contractions result in caudad movement of the intestinal luminal contents, either for absorption at more distal sites of the small or large intestine or for elimination in the stool. Peristaltic propulsion occurs as a result of the contraction of the circular muscle and relaxation of the longitudinal muscle in the propulsive or upstream segment, together with the relaxation of the circular muscle and contraction of the longitudinal muscle in the downstream receiving segment. Thus, circular smooth muscle in the small intestine participates in both segmental and peristaltic contractions. Parasympathetic innervation and the GI hormones gastrin and CCK promote intestinal motility; sympathetic innervation inhibits it. Human Physiology an Integrated Approach, 8e The patterns of electrical and mechanical activity differ in the fasting and fed states. In the fasting state, the small intestine is relatively quiescent but exhibits synchronized, rhythmic changes in both electrical and motor activity. The interdigestive myoelectric or migrating motor complex (MMC) is the term used to describe these rhythmic contractions of the small intestine that are observed in the fasting state. MMCs in humans occur at intervals of 90 to 120 minutes The slow propulsive contractions of the MMCs clear the small intestine of its residual content, including undigested food, bacteria, desquamated cells, and intestinal and pancreatic biliary secretions. MMCs usually originate in the stomach and often travel to the distal end of the ileum, but ~25% are initiated in the duodenum and proximal part of the jejunum. Feeding terminates MMCs and initiates the appearance of the fed motor pattern. Medical Physiology: A Cellular and Molecular Approach 2e If the small intestine wall becomes overdistended or irritated, a strong peristaltic rush may pass along the organ’s entire length. This movement sweeps the contents of the small intestine into the large intestine so quickly that water, nutrients, and electrolytes that would normally be absorbed are not. The result is diarrhea, characterized by more frequent defecation and watery stools. Prolonged diarrhea causes imbalances in water and electrolyte concentrations. Anatomy of the Small Intestine The anatomy of the small intestine facilitates secretion, digestion, and absorption by maximizing surface area. At the macroscopic level, the surface of the lumen is sculpted into fingerlike villi and deep crypts. Most absorption takes place along the villi while fluid and hormone secretion and cell renewal from stem cells occur in the crypts. On a microscopic level the apical surface of the enterocytes is modified into microvilli whose surfaces are covered with membrane-bound enzymes and a glycocalyx coat. The surface of the intestinal epithelium is called the brush border from the bristle-like appearance of the microvilli. Villi and crypts increase the effective surface area of the small intestine. Stem cells in the crypts produce new epithelial cells to replace those that die or are damaged. Most absorption occurs along the villi. Most fluid secretion occurs in the crypts. Human Physiology an Integrated Approach, 8e Most nutrients absorbed across the intestinal epithelium move into capillaries in the villi for distribution through the circulatory system. The exception is digested fats, most of which pass into lacteals of the lymphatic system. Venous blood from the digestive tract does not go directly back to the heart. Instead, it passes into the hepatic portal system. Hepatocytes contain a variety of enzymes, such as the cytochrome P450 isozymes, that metabolize drugs and xenobiotics and clear them from the bloodstream before they reach the systemic circulation. Human Physiology an Integrated Approach, 8e Secretions of the Small Intestine Located over the entire surface of the small intestine are small pits called crypts of Lieberkühn, These crypts lie between the intestinal villi. The surfaces of both the crypts and the villi are covered by an epithelium composed of two types of cells: a moderate number of goblet cells, which secrete mucus that lubricates and protects the intestinal surfaces; and a large number of enterocytes (brush border epithelial cells), which, in the crypts, secrete large quantities of water and electrolytes and, over the surfaces of adjacent villi, reabsorb the water and electrolytes along with the end products of digestion. Secretions of the Small Intestine Brunner's glands, are located in the wall of the first few centimeters of the duodenum, mainly between the pylorus of the stomach and the ampulla of Vater. These glands secrete large amounts of alkaline mucus in response to the following: (1) tactile or irritating stimuli on the duodenal mucosa; (2) vagal stimulation, which causes increased Brunner gland secretion concurrently with an increase in stomach secretion; and (3) gastrointestinal hormones, especially secretin. The function of the mucus secreted by Brunner’s glands is to protect the duodenal wall from digestion by the highly acidic gastric juice emptying from the stomach. Brunner's glands are inhibited by sympathetic stimulation. Digestion in Small Intestine Digestion is completed in the small intestine by the action of both pancreatic enzymes and enzymes at the brush border of the small intestinal mucosa Digestion by these brush border enzymes is referred to as membrane digestion. The enterocytes of the mucosa, especially those covering the villi, contain digestive enzymes that digest specific food substances while absorbed through the epithelium. These enzymes are as follows: (1) several peptidases for splitting small peptides into amino acids; (2) Four enzymes—sucrase, maltase, isomaltase, and lactase—for splitting disaccharides into monosaccharides; and (3) small amounts of intestinal lipase for splitting neutral fats into glycerol and fatty acids. Peptidases- split peptides into their constituent amino acids; Sucrase, maltase, and lactase - split the disaccharides sucrose, maltose, and lactose into the monosaccharides glucose, fructose, and galactose; Intestinal lipase - splits triglyceride into fatty acids and glycerol. Cytosolic peptidase Medical Physiology: A Cellular and Molecular Approach 2e General mechanisms of digestion and absorption. First, the substance (e.g., glucose) may not require digestion; the intestinal cells may absorb the nutrient as ingested. Second, a polymer (e.g., protein) may be digested in the lumen to its constituent monomers (e.g., amino acids) by pancreatic enzymes before absorption. Third, an oligomer (e.g., sucrose) is digested into its constituent monomers (e.g., monosaccharides) by brush-border enzymes before absorption. Fourth, an oligomer (e.g., oligopeptide) may be directly absorbed by the cell and then broken down into monomers (e.g., amino acids) inside the cell. Finally, a substance (e.g., TAG) may be broken down into its constituent components before absorption; the cell may then resynthesize the original molecule. Medical Physiology: A Cellular and Molecular Approach 2e Regulation of the Intestinal Phase The regulation of intestinal digestion and absorption comes primarily from signals that control motility and secretion. Sensors in the intestine trigger neural and endocrine reflexes that feedback to regulate the delivery rate of chyme from the stomach, and feed-forward to promote digestion, motility, and utilization of nutrients. The efferent control signals from the gut to the stomach and pancreas are both neural and hormonal: 1. Chyme entering the intestine activates the enteric nervous system, which then decreases gastric motility and secretion and slows gastric emptying. In addition, three hormones reinforce the “decrease motility” signal: secretin, cholecystokinin (CCK), and gastric inhibitory peptide (GIP) Medical Physiology: A Cellular and Molecular Approach 2e 2. Secretin is released by the presence of acidic chyme in the duodenum. Secretin inhibits acid production and decreases gastric motility. In addition, secretin stimulates the production of pancreatic bicarbonate to neutralize the acidic chyme that has entered the intestine. Medical Physiology: A Cellular and Molecular Approach 2e 3. CCK is secreted into the bloodstream if a meal contains fats. CCK also slows gastric motility and acid secretion. Because fat digestion proceeds more slowly than either protein or carbohydrate digestion, it is crucial that the stomach allows only small amounts of fat into the intestine at one time. Medical Physiology: A Cellular and Molecular Approach 2e 4. The incretin hormones GIP and glucagon-like peptide-1 (GLP-1) are released if the meal contains carbohydrates. Both hormones feed forward to promote insulin release by the endocrine pancreas, allowing cells to prepare for glucose that is about to be absorbed. They also slow the entry of food into the intestine by decreasing gastric motility and acid secretion. Medical Physiology: A Cellular and Molecular Approach 2e 5. The mixture of acid, enzymes, and digested food in chyme usually forms a hyperosmotic solution. Sensors in the intestine wall are sensitive to the osmolarity of the entering chyme. When stimulated by high osmolarity, the sensors inhibit gastric emptying in a reflex mediated by some unknown blood-borne substance. Medical Physiology: A Cellular and Molecular Approach 2e Large Intestine Diameter : 7 cm, length: 1.5 m The large intestine consists of the cecum, colon, rectum, and anal canal. The large intestine absorbs water and electrolytes from chyme remaining in the alimentary canal. It also forms and stores feces. Hole's essentials of human anatomy and physiology, 11th edition By the end of the ileum, only about 1.5 liters of unabsorbed chyme remain. The colon absorbs most of this volume so that normally only about 0.1 liter of water is lost daily in feces. Chyme enters the large intestine through the ileocecal valve. This is a tonically contracted region of muscular that narrows the opening between the ileum and the cecum, the initial section of the large intestine. The ileocecal valve relaxes each time a peristaltic wave reaches it. It also relaxes when food leaves the stomach as part of the gastroileal reflex. The wall of the ileum for several centimeters immediately upstream from the ileocecal valve has a thickened circular muscle called the ileocecal sphincter. This sphincter normally remains mildly constricted and slows the emptying of ileal contents into the cecum. However, immediately after a meal, a gastroileal reflex intensifies peristalsis in the ileum, and emptying of ileal contents into the cecum proceeds. The human large intestine has four primary functions: 1. The colon absorbs large quantities of fluid and electrolytes and converts the liquid content of ileocecal material to solid or semisolid stool. 2. The colon absorbs the short-chain fatty acids formed by the catabolism (or fermentation) of dietary carbohydrates that are not absorbed in the small intestine. The abundant colonic microflora accomplishes this fermentation. 3. The storage of colonic content represents a reservoir function of the large intestine. 4. The colon eliminates its contents in a regulated and controlled fashion, largely under voluntary control. To accomplish these important activities, the large intestine functionally acts as two distinct organs. The proximal (or ascending and transverse) part of the colon is the site where most of the fluid and electrolyte absorption occurs and where bacterial fermentation takes place. The distal (or descending and rectosigmoid) portion of the colon provides final desiccation, as well as reservoir function, and serves as a storage organ for colonic material before defecation. In contrast to the motor pattern in the small intestine, no distinct fasting and fed patterns of contractions are seen in the colon. Similarly to small-intestinal motor activity, colonic contractions are regulated by myogenic, neurogenic, and hormonal factors. Parasympathetic control of the proximal two-thirds of the colon is mediated by the vagus nerve, whereas parasympathetic control of the descending and rectosigmoid colon is mediated by pelvic nerves originating from the sacral spinal cord. The wall of the colon differs from that of the small intestine in that the muscularis of the large intestine has an inner circular layer but a discontinuous longitudinal muscle layer concentrated into three bands called the tenia coli. Contractions of the tenia pull the wall into bulging pockets called haustra {haustrum}. The mucosa of the colon has two regions, like that of the small intestine. The luminal surface lacks villi and appears smooth. It is composed of colonocytes and mucus-secreting goblet cells. The crypts contain stem cells that divide to produce new epithelium, as well as goblet cells, endocrine cells, and maturing colonocytes. Motility in the Large Intestine The proximal colon has two types of motor activity, nonpropulsive segmentation (mixing) and mass peristalsis (movement) Haustrations Segmentation contractions as in the small intestine, mix the contents of the large intestine, which aids in water absorption. As a haustrum fills with food residue, the distension stimulates the circular muscle to contract, which propels the luminal contents into the next haustrum. Mass Movement Forward movement is minimal during mixing contractions and depends primarily on a unique colonic contraction known as mass movement. A wave of contraction decreases the diameter of a segment of the colon and sends a substantial bolus of material forward. These contractions occur 3–4 times a day and are associated with eating and distension of the stomach through the gastrocolic reflex. Mass movement is responsible for the sudden distension of the rectum that triggers defecation. Initiation of Mass Movements by Gastrocolic and Duodenocolic Reflexes The appearance of mass movements after meals is facilitated by gastrocolic and duodenocolic reflexes. These reflexes result from distention of the stomach and duodenum. They occur either not at all or hardly at all when the extrinsic autonomic nerves to the colon have been removed; therefore, the reflexes almost certainly are transmitted by way of the autonomic nervous system. Irritation in the colon can also initiate intense mass movements. Defecation The “anal sphincter” consists of both an internal and an external sphincter. The internal sphincter has both circular and longitudinal smooth muscle and is under involuntary control. The external sphincter, which encircles the rectum, contains only striated muscle and is controlled by voluntary, conscious, or at least subconscious control; subconsciously, the external sphincter is usually kept continuously constricted unless conscious signals inhibit the constriction. The defecation reflex removes undigested feces from the body. Defecation resembles urination in that it is a spinal reflex triggered by distension of the organ wall. Most of the time, the rectum is empty of feces, but when feces are forced into the rectum by mass movements, stretching of the rectal wall initiates the defecation reflexes. One of these reflexes is an intrinsic reflex mediated by the local enteric nervous system in the rectal wall. When feces enter the rectum, distention of the rectal wall initiates afferent signals that spread through the myenteric plexus to initiate peristaltic waves in the descending colon, sigmoid, and rectum, forcing feces toward the anus. As the peristaltic wave approaches the anus, the internal anal sphincter is relaxed by inhibitory signals from the myenteric plexus; if the external anal sphincter is also consciously, and voluntarily relaxed at the same time, defecation occurs. Normally when the intrinsic myenteric defecation reflex is functioning by itself, it is relatively weak. To be effective in causing defecation, it usually must be fortified by another type of defecation reflex called a parasympathetic defecation reflex that involves the sacral segments of the spinal cord. When the nerve endings in the rectum are stimulated, signals are transmitted first into the spinal cord and then reflexly back to the descending colon, sigmoid, rectum, and anus via parasympathetic nerve fibers in the pelvic nerves. These parasympathetic signals greatly intensify the peristaltic waves and relax the internal anal sphincter, thus converting the intrinsic myenteric defecation reflex from a weak effort into a powerful process of defecation. Defecation signals entering the spinal cord initiate other effects, such as taking a deep breath, closure of the glottis, and contraction of the abdominal wall muscles to force the fecal contents of the colon downward, and at the same time they cause the pelvic floor to relax downward and pull outward on the anal ring to evaginate the feces. Constipation Constipation means slow movement of feces through the large intestine. Constipation is often associated with large quantities of dry, hard feces in the descending colon that accumulate because of excess absorption of fluid or insufficient fluid intake. Any pathology of the intestines that obstructs movement of intestinal contents, such as tumors, adhesions that constrict the intestines, or ulcers, can cause constipation. Constipation can also result from spasm of a small segment of the sigmoid colon. Motility normally is weak in the large intestine, so even a slight degree of spasm may cause serious constipation. After constipation has continued for several days and excess feces have accumulated above a spastic sigmoid colon, excessive colonic secretions often then lead to a day or so of diarrhea. After this, the cycle begins again, with repeated bouts of alternating constipation and diarrhea. Digestion and Absorption in the Large Intestine Numerous bacteria (intestinal flora) inhabiting the colon break down significant amounts of undigested complex carbohydrates and proteins through fermentation. The end products include lactate and short-chain fatty acids, such as butyric acid. Several of these products are lipophilic and can be absorbed by simple diffusion. The fatty acids, for example, are used by colonocytes as their preferred energy substrate. Colonic bacteria also produce significant amounts of absorbable vitamins, especially vitamin K. Intestinal gases, such as hydrogen sulfide, that escape from the gastrointestinal tract are a less useful product. Feces Feces include materials not digested or absorbed, plus water, electrolytes, mucus, shed intestinal cells, and bacteria. Usually, feces are about 75% water, and their color derives from bile pigments altered by bacterial action. Feces’ pungent odor results from a variety of compounds that bacteria produce. Human Physiology an Integrated Approach, 8e
