Physiology of Digestion and Absorption PDF

Physiology of Digestion and Absorption Hamidizad 1 Overview of the Digestive System – The Digestive System Consists of ; a) Long hollow muscular tube or canal or tract called gastrointestinal tract or (GIT): it is about 5 meters long b) Accessory glands: include:...

Physiology of Digestion and Absorption Hamidizad 1 Overview of the Digestive System – The Digestive System Consists of ; a) Long hollow muscular tube or canal or tract called gastrointestinal tract or (GIT): it is about 5 meters long b) Accessory glands: include: Salivary glands Liver and gall bladder Pancreas  The alimentary tract function (1) movement of food through the alimentary tract ‫الهضم‬ (2) secretion of digestive juices and digestion of the food (3) absorption of water, various electrolytes, and digestive products (4) circulation of blood through the gastrointestinal organs to carry away the absorbed substances (5) control of all these functions by local, nervous, and hormonal systems Overview of the Digestive movement of food through the alimentary tract; System (2) secretion of digestive juices and digestion of the food; (3) absorption of water, various electrolytes, and digestive products; (4) circulation of blood through the gastrointestinal organs to carry away the absorbed substances (5) control of all these functions by local, nervous, and hormonal systems.  Overview of the Digestive System – GIT consists of; Oral cavity or mouth Pharynx Esophagus Stomach Small intestine Large intestine Rectum Anus  Anatomy of wall of GIT (1) the serosa (2) longitudinal muscle layer (3) circular muscle layer (4) the submucosa, (5) the mucosa Gastrointestinal Smooth Muscle Functions as a Syncytium. The individual smooth muscle fibers in the gastrointestinal tract are 200 to 500 micrometers in length and 2 to 10 micrometers in diameter, and they are arranged in bundles of as many as 1000 parallel fibers. In the longitudinal muscle layer, the bundles extend longitudinally down the intestinal tract; in the circular muscle layer, they extend around the gut. Within each bundle, the muscle fibers are electrically connected with one another through large numbers of gap junctions that allow low-resistance movement of ions from one muscle cell to the next. Therefore, electrical signals that initiate muscle contractions can travel readily from one fiber to the next within each bundle but more rapidly along the length of the bundle than sideways. Each bundle of smooth muscle fibers is partly separated from the next by loose connective tissue, but the muscle bundles fuse with one another at many points, Therefore, each muscle layer functions as a syncytium; that is mien, when an action potential is elicited anywhere within the muscle mass, it generally travels in all directions in the muscle. Gap junctions The electrical synapse: In contrast, are characterized by direct open fluid channels that conduct electricity from one cell to the next. Most of these consist of small protein tubular structures called gap junctions that allow free movement of ions from the interior of one cell to the interior of the next. Gastrointestinal Smooth Muscle Functions as a Syncytium The smooth muscle of the gastrointestinal tract is excited by almost continual slow, intrinsic electrical activity along the membranes of the muscle fibers. This activity has two basic types of electrical waves: (1) slow waves and (2) spikes, Slow Waves. Most gastrointestinal contractions occur rhythmically, and this rhythm is determined mainly by the frequency of so-called “slow waves” of smooth muscle membrane potential. They are not action potentials Instead they are slow, undulating changes in the resting membrane potential. Their intensity usually varies between 5 and 15 millivolts, and their frequency ranges in different parts of the human gastrointestinal tract from 3 to 12 per minute: about 3 in the body of the stomach, as much as 12 in the duodenum, and about 8 or 9 in the terminal ileum. Therefore, the rhythm of contraction of the body of the stomach usually is about 3 per minute, of the duodenum about 12 per minute, and of the ileum 8 to 9 per minute. The precise cause of the slow waves is not completely understood, although they appear to be caused by complex interactions among the smooth muscle cells and specialized cells, called the interstitial cells of Cajal, that are believed to act as electrical pacemakers for smooth muscle cells. The slow waves usually do not by themselves cause muscle contraction in most parts of the gastrointestinal tract, except perhaps in the stomach. Instead, they mainly excite the appearance of intermittent spike potentials, and the spike potentials in turn actually excite the muscle contraction. Spike Potentials. The spike potentials are true action potentials. They occur automatically when the resting membrane potential of the gastrointestinal smooth muscle becomes more positive than about -40 millivolts (the normal resting membrane potential in the smooth muscle fibers of the gut is between -50 and -60 millivolts) In gastrointestinal smooth muscle fibers, the channels responsible for the action potentials (depolarization) are somewhat different; they allow especially large numbers of calcium ions to enter along with smaller numbers of sodium ions and therefore are called calcium-sodium channels. These channels are much slower to open and close than are the rapid sodium channels of large nerve fibers. The slowness of opening and closing of the calcium-sodium channels accounts for the long duration of the action potentials.  Changes in Voltage of the Resting Membrane Potential Changes in Voltage of the Resting Membrane Potential. Factors that depolarize the membrane—that is, make it more In addition to the slow waves and spike excitable: potentials, the baseline (1) stretching of the muscle, voltage level of the smooth muscle resting (2) stimulation by acetylcholine, membrane potential also can change. Under (3) Stimulation by parasympathetic nerves that secrete normal conditions, the acetylcholine at their endings resting membrane potential averages about (4) stimulation by several specific gastrointestinal hormones -56 millivolts, but multiple factors can Important factors that make the membrane potential change this level. When the potential becomes more negative—that is, hyperpolarize the membrane less negative, which is and make the muscle fibers less excitable: called depolarization of the membrane, the (1) the effect of norepinephrine or epinephrine on the muscle fibers become more excitable. When fiber membrane the potential becomes more negative, which is (2) stimulation of the sympathetic nerves that secrete mainly called hyperpolarization, the fibers become less norepinephrine at their endings. excitable. Neural Control of Gastrointestinal Function—Enteric Nervous System The gastrointestinal tract has a nervous system all its own called the enteric nervous system. beginning in the esophagus and extending all the way to the anus. The number of neurons in this enteric system is about 100 million. The enteric nervous system is composed mainly of two plexuses, shown in Figure 62–4: (1) an outer plexus lying between the longitudinal and circular muscle layers, called the myenteric plexus or Auerbach’s plexus, and (2) an inner plexus, called the submucosal plexus or Meissner’s plexus. The myenteric plexus controls mainly the gastrointestinal movements, and the submucosal plexus controls mainly gastrointestinal secretion and local blood flow. When the myenteric plexus is stimulated, its principal effects are ‫تقلصات على فترة زمنيه طويله‬ (1) increased tonic ‫كثافة‬ contraction, or “tone,” of the gut wall, (2) increased intensity of the rhythmical contractions, (3) slightly increased rate of the rhythm of contraction, (4) increased velocity of conduction of excitatory waves along the gut wall, causing more rapid movement of the gut peristaltic waves. The myenteric plexus should not be considered entirely excitatory because some of its neurons are inhibitory; their fiber endings secrete an inhibitory transmitter, possibly vasoactive intestinal polypeptide or some other inhibitory peptide. The resulting inhibitory signals are especially useful for inhibiting some of the intestinal sphincter muscles that impede movement of food along successive segments of the gastrointestinal tract, such as the pyloric sphincter, which controls emptying of the stomach into the duodenum, and the sphincter of the ileocecal valve, which controls emptying from the small intestine into the cecum. the extrinsic sympathetic and parasympathetic fibers that connect to both the myenteric and submucosal plexuses. Also shown in Figure 62–4 are sensory nerve endings that originate in the gastrointestinal epithelium or gut wall and send afferent fibers to both plexuses of the enteric system, as well as (1) to the prevertebral ganglia of the sympathetic nervous system, (2) to the spinal cord, and (3) in the vagus nerves all the way to the brain stem. These sensory nerves can elicit local reflexes within the gut wall itself and still other reflexes that are relayed to the gut from either the prevertebral ganglia or the basal regions of the brain. Gastrointestinal Reflexes 1) Reflexes that are integrated entirely within the gut wall enteric nervous system. 2) Reflexes from the gut to the prevertebral sympathetic ganglia and then back to the gastrointestinal tract. 3) Reflexes from the gut to the spinal cord or brain stem and then back to the gastrointestinal tract. The anatomical arrangement of the enteric nervous system and its connections with the sympathetic and parasympathetic systems support three types of gastrointestinal reflexes that are essential to gastrointestinal control. They are the following: Reflexes that are integrated entirely within the gut wall enteric nervous system. These include reflexes that control much gastrointestinal secretion, peristalsis, mixing contractions, local inhibitory effects, and so forth. 2. Reflexes from the gut to the prevertebral sympathetic ganglia and then back to the gastrointestinal tract. These reflexes transmit signals long distances to other areas of the gastrointestinal tract, such as signals from the stomach to cause evacuation of the colon (the gastrocolic reflex), signals from the colon and small intestine to inhibit stomach motility and stomach secretion (the enterogastric reflexes), and reflexes from the colon to inhibit emptying of ileal contents into the colon (the colono ileal reflex). 3. Reflexes from the gut to the spinal cord or brain stem and then back to the gastrointestinal tract. These include especially (1) reflexes from the stomach and duodenum to the brain stem and back to the stomach—by way of the vagus nerves—to control gastric motor and secretory activity; (2) pain reflexes that cause general inhibition of the entire gastrointestinal tract; and (3) defecation reflexes that travel from the colon and rectum to the spinal cord and back again to produce the powerful colonic, rectal, and abdominal contractions required for defecation (the defecation reflexes). Main Functions of Digestive Tract ❑4 major activities of GI tract: 1. Motility Propel ingested food from mouth toward rectum 2. Secretion of juices e.g. saliva Aid in digestion and absorption 3.Digestion Food broken down into absorbable molecules 4.Absorption Nutrients, electrolytes, and water are absorbed or transported from lumen of GIT to blood stream Main Functions of Digestive Tract  Functional Types of Movements in the Gastrointestinal Tract – Propulsive movements (Peristalsis) which cause food to move forward along the tract – Mixing movements (segmentation) which keep the intestinal contents thoroughly mixed at all times Propulsive Movements—Peristalsis The basic propulsive movement of the gastrointestinal tract is peristalsis. A contractile ring appears around the gut and then moves forward; Peristalsis is an inherent property of many syncytial smooth muscle tubes; stimulation at any point in the gut can cause a contractile‫مراره‬ring ‫صفراء‬ to appear ‫قناة الغدية‬ in the circular ‫حالب‬ muscle, and this ring then spreads along the gut tube. (Peristalsis also occurs in the bile ducts, glandular ducts, ureters, and many other smooth muscle tubes of the body.) The usual stimulus for intestinal peristalsis is distention of the gut. That is, if a large amount of food collects at any point in the gut, the stretching of the gut ‫انتفاخ‬ wall stimulates the enteric nervous system to contract the gut wall 2 to 3 centimeters behind this point, and a contractile ring appears that initiates a peristaltic ‫تهيج‬ movement. Other stimuli that can initiate peristalsis include chemical or physical irritation of the epithelial lining in the gut. Also, strong parasympathetic nervous signals to the gut will elicit strong peristalsis. Function of the Myenteric Plexus in Peristalsis. Peristalsis occurs only weakly or not at all in any portion of the gastrointestinal tract that has congenital absence of the myenteric plexus. Also, it is greatly depressed or completely blocked in the entire gut when a person is treated with atropine to paralyze the cholinergic nerve endings of the myenteric plexus. Therefore, effectual peristalsis requires an active myenteric plexus. The peristalsis movement direction is from the oral side of the distended segment and moves toward the distended segment, pushing the intestinal contents in the anal direction for 5 to 10 centimeters before dying out. Mixing Movements Mixing movements differ in different parts of the alimentary tract. In some areas, the peristaltic contractions themselves cause most of the mixing. This is especially true when forward progression of the intestinal contents is blocked by a sphincter, so that a peristaltic wave can then only churn the intestinal contents, rather than propelling them forward. At other times, local intermittent constrictive contractions occur every few centimeters in the gut wall. These constrictions usually last only 5 to 30 seconds; then new constrictions occur at other points in the gut, thus “chopping” and “shearing” the contents first here and then there. These peristaltic and constrictive movements are modified in different parts of the gastrointestinal tract for proper propulsion and mixing. Hormonal Control of Gastrointestinal Motility Gastrin (1) stimulation of gastric acid secretion (2) Stimulation of growth of the gastric mucosa Cholecystokinin 1) strongly contracts the gallbladder, expelling bile into the small intestine 2) inhibits stomach contraction moderately 3) slows the emptying of food from the stomach several hormones for controlling gastrointestinal secretion. Most of these same hormones also affect motility in some parts of the gastrointestinal tract. Gastrin is secreted by the “G” cells of the antrum of the stomach in response to stimuli associated with ingestion of a meal, such as distention of the stomach, the products of proteins, and gastrin releasing peptide, which is released by the nerves of the gastric mucosa during vagal stimulation. The primary actions of gastrin are (1) stimulation of gastric acid secretion and (2) stimulation of growth of the gastric mucosa. Cholecystokinin is secreted by “I” cells in the mucosa of the duodenum and jejunum mainly in response to digestive products of fat, fatty acids, and mon oglycerides in the intestinal contents. This hormone strongly contracts the gallbladder, expelling bile into the small intestine where the bile in turn plays important roles in emulsifying fatty substances, allowing them to be digested and absorbed. Cholecystokinin also inhibits stomach contraction moderately. Therefore, at the same time that this hormone causes emptying of the gallbladder, it also slows the emptying of food from the stomach to give adequate time for digestion of the fats in the upper intestinal tract. Hormonal Control of Gastrointestinal Motility ❑Secretin Promote pancreatic secretion of bicarbonate ❑Gastric inhibitory peptide (GIP) Decreasing motor activity of the stomach ❑Motilin this hormone is to increase gastrointestinal motility in fasting Secretin was the first gastrointestinal hormone discovered and is secreted by the “S” cells in the mucosa of the duodenum in response to acidic gastric juice emptying into the duodenum from the pylorus of the stomach. Secretin has a mild effect on motility of the gastrointestinal tract and acts to promote pancreatic secretion of bicarbonate which in turn helps to neutralize the acid in the small intestine. Gastric inhibitory peptide is secreted by the mucosa of the upper small intestine, mainly in response to fatty acids and amino acids but to a lesser extent in response to carbohydrate. It has a mild effect in decreasing motor activity of the stomach and therefore slows emptying of gastric contents into the duodenum when the upper small intestine is already overloaded with food products. Motilin is secreted by the upper duodenum during fasting, and the only known function of this hormone is to increase gastrointestinal motility. Motilin is released cyclically and stimulates waves of gastrointestinal motility called inter digestive myoelectric complexes that move through the stomach and small intestine every 90 minutes in a fasted person. Motilin secretion is inhibited after ingestion by mechanisms that are not fully understood.  Motility of the GIT 1. Motility in the mouth 2 types; a) Chewing or Mastication: It is reflex in nature Significance: 1. Breaks the food into small pieces to be easily swallowed 2. Expose food to salivary amylase enzyme, which begins digestion of starch 3. Help digestion of all types of food especially cellulose containing food e.g. vegetables  b) Swallowing: Swallowing is the transport of food from mouth to Steps: stomach It consists of 3 phases or steps Buccal Phase: food is pushed back into pharynx from mouth Pharyngeal Phase: food pass through pharynx to esophagus Oesophageal Phase: food pass through esophagus to stomach by peristaltic movements  2. Motility of Esophagus The esophagus is 25 cm tube It is guarded by 2 sphincters; 1. Upper esophageal sphincter prevents air from entering the GIT 2. Lower esophageal sphincter prevents gastric contents from Re-entering the esophagus from the stomach Esophageal peristalsis sweeps down the esophagus Function of the Lower Esophageal Sphincter (Gastroesophageal Sphincter). At the lower end of the esophagus, extending upward about 3 centimeters above its juncture with the stomach, the esophageal circular muscle functions as a broad lower esophageal sphincter, also called the gastroesophageal sphincter. This sphincter normally remains tonically constricted with an intraluminal pressure at this point in the esophagus of about 30 mm Hg, in contrast to the mid portion of the esophagus, which normally remains relaxed. When a peristaltic swallowing wave passes down the esophagus, there is “receptive relaxation” of the lower esophageal sphincter ahead of the peristaltic wave, which allows easy propulsion of the swallowed food into the stomach. The stomach secretions are highly acidic and contain many proteolytic enzymes. The esophageal mucosa, except in the lower one eighth of the esophagus, is not capable of resisting for long the digestive action of gastric secretions. Fortunately, the tonic constriction of the lower esophageal sphincter helps to prevent significant reflux of stomach contents into the esophagus except under very abnormal conditions.  3. Motility of Stomach The stomach consists of fundus, body and pylorus Proximal area (fundus and body) has a thin wall and contracts weakly and infrequently → holds large volumes of food ( to store food ) because of receptive relaxation Distal area (pylorus) has thick Wall with strong and frequent Peristaltic contractions that mix and propel food into the duodenum. Also, distal area is responsible for gastric emptying into duodenum Anatomically, the stomach is usually divided into two major parts: (1) the body and (2) the antrum. Physiologically, it is more appropriately divided into (1) the “orad” portion, comprising about the first two thirds of the body, and (2) the “caudad” portion, comprising the remainder of the body plus the antrum. Storage Function of the Stomach: Normally, when food stretches the stomach, a “vagovagal reflex” from the stomach to the brain stem and then back to the stomach reduces the tone in the muscular wall of the body of the stomach so that the wall bulges progressively outward, accommodating greater and greater quantities of food up to a limit in the completely relaxed stomach of 0.8 to 1.5 liters. The pressure in the stomach remains low until this limit is approached. Motor Functions of the Stomach (1)Storage of large quantities of food until the food can be processed in the stomach, duodenum, and lower intestinal tract (2)Mixing of this food with gastric secretions until it forms a semifluid mixture called chyme (3) Slow emptying of the chyme from the stomach into the small intestine at a rate suitable for proper digestion and absorption by the small intestine. The digestive juices of the stomach are secreted by gastric glands, which are present in almost the entire wall of the body of the stomach except along a narrow strip on the lesser curvature of the stomach. These secretions come immediately into contact with that portion of the stored food lying against the mucosal surface of the stomach. As long as food is in the stomach, weak peristaltic constrictor waves, called mixing waves, begin in the mid- to upper portions of the stomach wall and move toward the antrum about once every 15 to 20 seconds. As the constrictor waves progress from the body of the stomach into the antrum, they become more intense, some becoming extremely intense and providing powerful peristaltic action potential–driven constrictor rings that force the antral contents under higher and higher pressure toward the pylorus. Chyme. After food in the stomach has become thoroughly mixed with the stomach secretions, the resulting mixture that passes down the gut is called chyme. The degree of fluidity of the chyme leaving the stomach depends on the relative amounts of food, water, and stomach secretions and on the degree of digestion that has occurred. The appearance of chyme is that of a murky semifluid or paste. Role of the Pylorus in Controlling Stomach Emptying. The distal opening of the stomach is the pylorus. Here the thickness of the circular wall muscle becomes 50 to 100 per cent greater than in the earlier portions of the stomach antrum, and it remains slightly tonically contracted almost all the time. Therefore, the pyloric circular muscle is called the pyloric sphincter. Despite normal tonic contraction of the pyloric sphincter, the pylorus usually is open enough for water and other fluids to empty from the stomach into the duodenum with ease. Conversely, the constriction usually prevents passage of food particles until they have become mixed in the chyme to almost fluid consistency. The degree of constriction of the pylorus is increased or decreased under the influence of nervous and humoral reflex signals from both the stomach and the duodenum.  Gastric Factors That Promote Emptying Effect of Gastric Food Volume on Rate of Emptying. Increased food volume in the stomach promotes increased emptying from the stomach. Effect of the Hormone Gastrin on Stomach Emptying. Most important, it seems to enhance the activity of the pyloric pump. Thus, it, too, probably promotes stomach emptying. Inhibitory Effect of Enterogastric Nervous Reflexes from the Duodenum. When food enters the duodenum, multiple nervous reflexes are initiated from the duodenal wall that pass back to the stomach to slow or even stop stomach emptying if the volume of chyme in the duodenum becomes too much. These reflexes are mediated by three routes: directly from the duodenum to the stomach through the enteric nervous system in the gut wall, (2) through extrinsic nerves that go to the prevertebral sympathetic ganglia and then back through inhibitory sympathetic nerve fibers to the stomach (3) probably to a slight extent through the vagus nerves all the way to the brain stem, where they inhibit the normal excitatory signals transmitted to the stomach through the vagi. All these parallel reflexes have two effects on stomach emptying: first, they strongly inhibit the “pyloric pump” propulsive contractions, and second, they increase the tone of the pyloric sphincter. The types of factors that are continually monitored in the duodenum and that can initiate enterogastric inhibitory reflexes include the following: 1. The degree of distention of the duodenum 2. The presence of any degree of irritation of the duodenal mucosa 3. The degree of acidity of the duodenal chyme 4. The degree of osmolality of the chyme 5. The presence of certain breakdown products in the chyme, especially breakdown products of proteins and perhaps to a lesser extent of fats The enterogastric inhibitory reflexes are especially sensitive to the presence of irritants and acids in the duodenal chyme, and they often become strongly activated within as little as 30 seconds. Hormonal Feedback from the Duodenum Inhibits Gastric Emptying ❖ Cholecystokinin (CCK), which is released from the mucosa of the jejunum in response to fatty substances in the chyme is the most potent feedback inhibition of the stomach. ❖Secretin is released mainly from the duodenal mucosa in response to gastric acid passed from the stomach through the pylorus acts as an inhibitor ❖ GIP has a general but weak effect of decreasing gastrointestinal motility.  4. Motility of Small intestine Types: Two basic motility patterns exist segmentation and peristalsis Significance: Motility of the small intestine serves 3 functions: 1. Mixing contents with enzymes and other secretions → help digestion 2. Maximizing exposure of the contents to membranes of intestinal cells → help absorption and digestion. 3. Propulsion of contents into the large intestine. Segmentation movements Control of Peristalsis by Nervous and Hormonal Signals. Peristaltic activity of the small intestine is greatly increased after a meal. This is caused partly by the beginning entry of chyme into the duodenum causing stretch of the duodenal wall, but also by a so-called gastroenteric reflex that is initiated by distention of the stomach and conducted principally through the myenteric plexus from the stomach down along the wall of the small intestine. In addition to the nervous signals that may affect small intestinal peristalsis, several hormonal factors also affect peristalsis. They include gastrin, CCK, insulin, motilin, and serotonin, all of which enhance intestinal motility and are secreted during various phases of food processing. Conversely, secretin and glucagon inhibit small intestinal motility. The physiologic importance of each of these hormonal factors for controlling motility is still questionable. The function of the peristaltic waves in the small intestine is not only to cause progression of chyme toward the ileocecal valve but also to spread out the chyme along the intestinal mucosa. As the chyme enters the intestines from the stomach and elicits peristalsis, this immediately spreads the chyme along the intestine; and this process intensifies as additional chyme enters the duodenum. On reaching the ileocecal valve, the chyme is sometimes blocked for several hours until the person eats another meal; at that time, a gastroileal reflex intensifies peristalsis in the ileum and forces the remaining chyme through the ileocecal valve into the cecum of the large intestine. 5. Motility of Large intestine or colon Types: Include : a) Segmentation in the large intestine causes the contents to be continuously mixed b) Mass movement propels the contents of one segment of the large intestine into the next downstream segment. c) Defecation involves involuntary reflexes and voluntary reflexes → evacuation of colonic content through anal canal Movements of the Colon The principal functions of the colon are absorption of water and electrolytes from the chyme to form solid feces (2) storage of fecal matter until it can be expelled Mixing Movements—“Haustrations.” In the same manner that segmentation movements occur in the small intestine, large circular constrictions occur in the large intestine. At each of these constrictions, about 2.5 centimeters of the circular muscle contracts, sometimes constricting the lumen of the colon almost to occlusion. At the same time, the longitudinal muscle of the colon, which is aggregated into three longitudinal strips called the teniae coli, contracts. These combined contractions of the circular and longitudinal strips of muscle cause the unstimulated portion of the large intestine to bulge outward into baglike sacs called haustrations. Each haustration usually reaches peak intensity in about 30 seconds and then disappears during the next 60 seconds. They also at times move slowly toward the anus during contraction, especially in the cecum and ascending colon, and thereby provide a minor amount of forward propulsion of the colonic contents. After another few minutes, new haustral contractions occur in other areas nearby. Therefore, the fecal material in the large intestine is slowly dug into and rolled over in much the same manner that one spades the earth. In this way, all the fecal material is gradually exposed to the mucosal surface of the large intestine, and fluid and dissolved substances are progressively absorbed until only 80 to 200 milliliters of feces are expelled each day Propulsive Movements—“Mass Movements.” Much of the propulsion in the cecum and ascending colon results from the slow but persistent haustral contractions, requiring as many as 8 to 15 hours to move the chyme from the ileocecal valve through the colon, while the chyme itself becomes fecal in quality, a semisolid slush instead of semifluid. From the cecum to the sigmoid, mass movements can, for many minutes at a time, take over the propulsive role. These movements usually occur only one to three times each day, in many people especially for about 15 minutes during the first hour after eating breakfast. A mass movement is a modified type of peristalsis characterized by the following sequence of events: First, a constrictive ring occurs in response to a distended or irritated point in the colon, usually in the transverse colon. Then, rapidly, the 20 or more centimeters of colon distal to the constrictive ring lose their haustrations and instead contract as a unit, propelling the fecal material in this segmenten masse further down the colon. The contraction develops progressively more force for about 30 seconds, and relaxation occurs during the next 2 to 3 minutes. Then, another mass movement occurs, this time perhaps farther along the colon. A series of mass movements usually persists for 10 to 30 minutes. Then they cease but return perhaps a half day later. When they have forced a mass of feces into the rectum, the desire for defecation is felt. Defecation Most of the time, the rectum is empty of feces. This results partly from the fact that a weak functional sphincter exists about 20 centimeters from the anus at the juncture between the sigmoid colon and the rectum. There is also a sharp angulation here that contributes additional resistance to filling of the rectum. When a mass movement forces feces into the rectum, the desire for defecation occurs immediately, including reflex contraction of the rectum and relaxation of the anal sphincters. Continual dribble of fecal matter through the anus is prevented by tonic constriction of (1) an internal anal sphincter, a several-centimeters-long thickening of the circular smooth muscle that lies immediately inside the anus, and (2) an external anal sphincter, composed of striated voluntary muscle that both surrounds the internal sphincter and extends distal to it. The external sphincter is controlled by nerve fibers in the pudendal nerve, which is part of the somatic nervous system and therefore is under voluntary, conscious or at least subconscious control; subconsciously, the external sphincter is usually kept continuously constricted unless conscious signals inhibit the constriction. Defecation Reflexes. Ordinarily, defecation is initiated by defecation reflexes. One of these reflexes is an intrinsic reflex mediated by the local enteric nervous system in the rectal wall. This can be described as follows: 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, voluntarily relaxed at the same time, defecation occurs. The intrinsic myenteric defecation reflex functioning by itself normally is relatively weak. To be effective in causing defecation, it usually must be fortified by another type of defecation reflex, 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 by way of parasympathetic nerve fibers in the pelvic nerves. These parasympathetic signals greatly intensify the peristaltic waves as well as relax the internal anal sphincter, thus converting the intrinsic myenteric defecation reflex from a weak effort into a powerful process of defecation that is sometimes effective in emptying the large bowel all the way from the splenic flexure of the colon to the anus.  Secretory Functions (Secretions) of GIT 31  Secretions of GIT The total volume of GIT secretions is about 6-8 L/day Secretions arise from specialized cells lining the GI tract , the pancreas , liver and gallbladder GI secretions function to lubricate (water and mucus) protect (mucus), sterilize (HCl), neutralize (HCO 3 --), and digest (enzymes)  Secretions of GIT in Mouth Three pairs of glands Parotid Sublingual Submandibular Functions of saliva 1.Lubricates, cleans oral cavity 2. Dissolves chemicals 3. Suppresses bacterial growth 4. Digest starch by amylase GIT secretions in Stomach GIT secretions in Stomach  Function of Gastric HCL – 1. Activates pepsinogen into pepsins – 2. Provides optimum for pH for action of pepsins – 3. Denatures protein denaturation help its digestion – 4. Kills bacteria in food – 5. Help Fe 2+ , Ca 2+ absorption – 6. Promotes pancreatic, small intestinal and bile secretion Function of pepsins  Function of mucous and intrinsic factor Mucus secretion Soluble and insoluble mucus are secreted by cells of the stomach. Soluble mucus mixes with the contents of the stomach and helps to lubricate chyme. Insoluble mucus forms a protective barrier against the high acidity of the stomach content. Intrinsic Factor Help absorption of vitamin B12 Function of mucous and intrinsic factor Regulation of Gastric Secretion  Pancreas has 2 functions a) Endocrine functions : secretes insulin and Pancreases glucagon from islets of Langerhans b) Exocrine function : secretion of pancreatic juice It has 2 components : aqueous and enzymatic Aqueous component (contains HCO 3 ) is important for neutralizing stomach acid in the duodenum so pancreatic enzymes can function properly Enzymatic component is essential for the proper digestion and absorption of carbohydrates, fats, and proteins Pancreatic enzymes include trypsin, chemo trypsin, lipase, and amylase Pancreatic proteolytic enzymes include; Trypsin, chymotrypsin and Functions of pancreatic juice carboxypeptidases, these enzymes are activated by enzymes enterokinase and trypsin. Pancreatic amylase hydrolyzes starch, glycogen and most carbohydrates (except cellulose). Lipase, cholesterol esterase and phospholipase are secreted to digest fats. Acetylcholine and cholecystokinin increase pancreatic enzyme secretions. Secretin stimulates the secretion of high amounts of bicarbonate (HCO3).  Bile plays a very important role in the digestion and absorption of fats. Also, bile is important for the elimination of several waste products, Liver especially bilirubin and excess amounts of cholesterol. About 94% of the bile solutes secreted into the digestive tract are reabsorbed. Cholesterol deposition in the bile can cause cholesterol gallstones, which can be dangerous if they enter the bile ducts. Functions of the Liver: 1) Metabolic regulation Store absorbed nutrients, vitamins Release nutrients as needed 2) Hematological regulation Plasma protein production Remove old RBCs 3) Production of bile Required for fat digestion and absorption  Small intestine Secretion Located over the entire surface of the small intestine are small pits called crypts of Lieberkühn, one of which is illustrated in Figure 64–13.These crypts lie between the intestinal villi. The surfaces of both the crypts and the villi are covered by an ❑ A large group of mucous glands called Brunner's epithelium composed of two types of cells: glands secrete alkaline mucus. (1) a moderate number of goblet cells, which secrete mucus that lubricates and protects ❑ Throughout the surface of the small intestine, therethe intestinal surfaces, and (2) a large number of enterocytes, which, in the crypts, are small pits called crypts of Lieberkühn, which secrete large quantities of water and electrolytes and, over the surfaces of are located between the intestinal villi. adjacent villi, reabsorb the water and ❑ Goblet cells secrete mucus. electrolytes along with end products of digestion. ❑ Enterocytes secrete water and electrolytes, which are reabsorbed in the villi. ❑ Enterocytes of the small intestine contain several peptidase, sucrase, maltase, lactase and lipase enzymes. ❑ Secretin and cholecystokinin increase small intestinal secretions.  Secretion of large intestine The mucosa of the large intestine, like that of the small intestine, has many crypts of Lieberkühn; however, unlike the small intestine, there are no villi. The epithelial cells contain almost no enzymes. Instead, they consist mainly of mucous cells that secrete only mucus. 1.Colonic alkaline secretion to neutralize acids produced by intestinal bacteria 2. Secretion of mucous for protection, lubrication of fecal matter 3. Vitamin B and K absorption made from bacterial flora in colon  Digestion and Absorption 45  Digestion and Absorption Digestion is a process essential for the conversion of food into a small and simple form. Mechanical digestion by mastication and swallowing Chemical digestion by enzymes Absorption is the process of transporting small molecules from the lumen of the gut into blood stream or lymphatic vessel  Small intestine is primary site for Intestinal Mucosa digestion and absorption of food. Digestion occurs in the GI lumen by secreted enzymes and on surface of enterocytes by membrane bound enzymes Absorption occurs by simple diffusion , facilitated diffusion , active transport , endocytosis , and paracellular transport Surface area of small intestine is greatly increased by extensive folding and the projection of fingerlike villi covered with microvilli  Digestion and Absorption of CHO Salivary petialin enzyme is secreted by parotid glands and can hydrolyze starch. But the food remains in the mouth only for a short time and the amylase of the saliva is deactivated by the stomach acid. Pancreatic juice contains stronger amylase and breaks starch into maltose and glucose. Then enterocytes of the small intestine convert disaccharides into monosaccharides by secreting lactase, sucrase and maltase enzymes and prepare them for absorption. Digestion and absorption of proteins Pepsin stomach enzyme breaks down collagen in meat. Pepsin only starts the process of protein digestion and usually performs 10-20% of the total protein digestion, and most of the protein digestion is done in the duodenum and jejunum under the influence of pancreatic juice proteolytic enzymes, which include: trypsin, chymotrypsin, Carboxy peptidase. Trypsin and chymotrypsin break down protein molecules into small polypeptides. Then carboxy polypeptidase converts polypeptides into amino acids. 99% of amino acids and the rest are transferred in the form of dipeptide and tripeptide from the membrane of the villi into enterocytes. (It is carried out as a transfer with sodium.) Absorption of proteins ❑The whole proteins by endocytosis absorb ❑Amino acids and Di and tripeptides by Na dependent Secondary active transport  Digestion of fats Triglycerides are the most abundant fats in the diet. A small amount of triglycerides in the stomach is digested by lipase (less than 10%) and the complete digestion of fat is done in the small intestine. Dietary fats are converted into fat emulsions (small fat particles) by bile. The most important fat digestion enzyme is pancreatic lipase. In addition, the enterocytes of the small intestine contain a small amount of intestinal lipase Absorption of Lipids Fatty acids and monoglycerides are soluble in the lipid membrane of enterocytes and are transported by simple diffusion. After entering the enterocytes, they are again transformed into triglycerides and are mainly transported in the chylomicrons of the lymph and then go to the blood circulation.  Absorption of Water Water is completely transported through the intestinal membrane by the diffusion method and follows the laws of osmosis. 54 Movements of the Colon The principal functions of the colon are absorption of water and electrolytes from the chyme to form solid feces (2) storage of fecal matter until it can be expelled Mixing Movements—“Haustrations.” In the same manner that segmentation movements occur in the small intestine, large circular constrictions occur in the large intestine. At each of these constrictions, about 2.5 centimeters of the circular muscle contracts, sometimes constricting the lumen of the colon almost to occlusion. At the same time, the longitudinal muscle of the colon, which is aggregated into three longitudinal strips called the teniae coli, contracts. These combined contractions of the circular and longitudinal strips of muscle cause the unstimulated portion of the large intestine to bulge outward into baglike sacs called haustrations. Each haustration usually reaches peak intensity in about 30 seconds and then disappears during the next 60 seconds. They also at times move slowly toward the anus during contraction, especially in the cecum and ascending colon, and thereby provide a minor amount of forward propulsion of the colonic contents. After another few minutes, new haustral contractions occur in other areas nearby. Therefore, the fecal material in the large intestine is slowly dug into and rolled over in much the same manner that one spades the earth. In this way, all the fecal material is gradually exposed to the mucosal surface of the large intestine, and fluid and dissolved substances are progressively absorbed until only 80 to 200 milliliters of feces are expelled each day Propulsive Movements—“Mass Movements.” Much of the propulsion in the cecum and ascending colon results from the slow but persistent haustral contractions, requiring as many as 8 to 15 hours to move the chyme from the ileocecal valve through the colon, while the chyme itself becomes fecal in quality, a semisolid slush instead of semifluid. From the cecum to the sigmoid, mass movements can, for many minutes at a time, take over the propulsive role. These movements usually occur only one to three times each day, in many people especially for about 15 minutes during the first hour after eating breakfast. A mass movement is a modified type of peristalsis characterized by the following sequence of events: First, a constrictive ring occurs in response to a distended or irritated point in the colon, usually in the transverse colon. Then, rapidly, the 20 or more centimeters of colon distal to the constrictive ring lose their haustrations and instead contract as a unit, propelling the fecal material in this segmenten masse further down the colon. The contraction develops progressively more force for about 30 seconds, and relaxation occurs during the next 2 to 3 minutes. Then, another mass movement occurs, this time perhaps farther along the colon. A series of mass movements usually persists for 10 to 30 minutes. Then they cease but return perhaps a half day later. When they have forced a mass of feces into the rectum, the desire for defecation is felt.