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Digestive System  |  Digestive Chart

Processes associated with the digestive system: ingestion - intake of food mastication- chewing, a component of physical digestion deglutition - swallowing propulsion - movement of materials along the alimentary canal. Occurs mostly in the alimentary canal as muscular movements producing segmentation and peristalsis. digestion - consists of physical (mechanical) digestion, and chemical digestion. Physical digestion is the reduction in bulk and increase in surface area of ingested food. Chemical digestion is enzymatic hydrolysis in which large complex molecules are broken down to their subunits. Physical digestion makes chemical digestion possible. secretion - the release of substances from cells in the digestive tract, e.g. mucus, enzymes, hormones, etc. absorption - the transport of digestive endproducts into the blood or the lymph. defecation - the removal of waste from the GI tract including undigested materials, exfoliated cells and bacteria. exfoliation - the constant shedding of the mucosal lining cells and their replacement by mitosis

The structure of the alimentary canal. (See Figure 24.6) The alimentary canal is the continuous tube stretching from the mouth to the anus. Components of this tube, the various organs of the system, are specialized to perform particular functions. The stomach and intestines are commonly referred to as the GI (gastrointestinal) tract. The alimentary canal is composed of four layers, each layer typically composed of certain tissues. But these layers can vary somewhat within the canal. mucosa - this is the lining tissue, mostly made of simple columnar epithelium (the mucosa of the esophagus is non-keratinized stratified squamous epithelium). Goblet cells within this layer secrete mucus for lubrication and protection and other cells may secrete enzymes, hormones etc. The lining through much of the alimentary canal exfoliates on a 3 to 5 day cycle. The gastrointestinal mucosa is also responsible for absorption of digestive endproducts. Beneath the epithelial surface is a connective-like component called the lamina propria. This layer contains blood and lymph capillaries for absorption. The boundary of the mucosa is the muscularis mucosae, the "muscle of the mucosa" which contracts to increase exposure of the mucosal lining to contents of the alimentary canal. The submucosa - this layer lies beneath the mucosa and is basically areolar connective tissue containing major blood vessels, nerves, and lymph nodes serving the alimentary canal. The submucosal nerve plexus controls the function of mucosal cells and digestive functions. The muscularis (or muscularis externae) - this is mostly smooth muscle (the esophagus has partly skeletal muscle) in two or three layers. In most of the GI tract two layers exist, the longitudinal smooth muscle layer and the circular or transverse smooth muscle layer. The circular layer squeezes to produce segments in the intestines, while the longitudinal layer causes the repeated shortening and lengthening called peristalsis (See Figure 24.3). Segmentation contractions are mostly mixing actions, but work together with peristalsis in propulsion. The serosa or fibroserous layer - this is the covering, a serous membrane in the portions of the alimentary canal in the peritoneal cavity and a fibrous covering in portions not in the peritoneal cavity or considered retroperitoneal. (See Figure 24.5)The serosa is continuous with the mesenteries which connect portions of the alimentary canal together

Organs and Regions of the Alimentary Canal: For an overview of the information covered and the chart used in class see [Digestive Chart]. The mouth: The mucosa of the mouth is composed of mostly non-keratinized stratified squamous epithelium. This mucosa continues through the esophagus. Three salivary glands on each side, plus buccal glands in the mucosa, provide the fluid known as saliva. Saliva contains water, salts, mucin, serous fluid, lysozyme, IgA, growth factors, and amylase. The three glands (See Figure 24.9) (parotid, submandibular, and sublingual) produce varying amounts of these components. The pH of this fluid is from 6.35 to 6.85, supporting the action of salivary amylase to begin the breakdown of polysaccharides to shorter chains. The action does not normally progress very far due to the short exposure to active enzyme. Chewing is a form of mechanical digestion which reduces the bulk of the food and, especially, exposes it to the enzyme. The bolus of food is swallowed in a process called deglutition which begins as voluntary and becomes involuntary (See Figure 24.13). At first the bolus is lodged on the tongue and pushed voluntarily into the pharynx. Then pharyngeal muscles contract pushing the bolus into the esophagus where peristalsis begins. Peristaltic waves move food down the esophagus into the stomach.

The esophagus: The esophagus is about 10" long and is also lined with non-keratinized stratified squamous epithelium. (See Figure 24.12) Esophageal glands located in the submucosa produce mucus for lubrication. The first third of the muscularis is skeletal, the last third is smooth muscle, and the middle of the esophagus is mixed smooth and skeletal muscle. Peristalsis begins in the esophagus and moves the bolus into the stomach. The muscle at the lower end of the esophagus remains contracted until the bolus arrives, then briefly relaxes to allow the bolus to pass, then tonically contracts again. Although traditionally referred to as the cardiac sphincter (now called the gastroesophageal region) of the stomach, there is no structural sphincter or valve in this region, and the region has nothing to do with the heart.

The stomach: The stomach is composed of several regions and structures - 1) The gastroesophageal region (a.k.a. cardia) mentioned above. 2) The fundus, the blind portion of the stomach above its junction with the esophagus. This portion is thin walled compared to the rest of the stomach and has few secretory cells. As the bolus of food enters this area first some action of salivary amylase may continue briefly. 3) The body of the stomach. This is where extensive gastric pits are located which possess the secretory cells of the stomach. 4) The pylorus. This narrowed region leads through the pyloric sphincter into the duodenum.

Rugae are the extensive folds in the stomach lining. These folds can stretch to accommodate an increase in stomach volume with consumption of a meal. 3-layered muscularis - an oblique layer in addition to the longitudinal and transverse layers. The three layers produce a churning and liquefying effect on the chyme in the stomach. Gastric pits increase the surface mucosa for secretion and absorption. Specialized columnar epithelial cells release enzymes and other substances: zymogen (chief) cells release pepsinogen and parietal cells release hydrochloric acid.  [IMPORTANT NOTE: Actually these cells secrete H+, derived from the same chemical reaction of CO2 and water which produces carbonic acid in the blood. The bicarbonate ions are retained and transported into the blood and the chloride ions are exchanged for them and pass into the stomach.]  The H+ causes activation of the pepsinogen to produce the protease pepsin. Mucous neck cells and mucous surface cells (there are no true goblet cells in the stomach) produce an alkaline mucus which helps protect the lining from the acidity, which in the stomach reaches a pH from 1.5 to 3.5. Enteroendocrine cells produce a number of hormone substances including gastrin, histamine, endorphins, serotonin and somatostatin. Cells lining the gastric pits are arranged in circular acini in the stomach called gastric glands. These glands are found throughout the stomach and vary from one area to another with regard to their complement of cells.

Processes occurring in the stomach: 1) Storage - the stomach allows a meal to be consumed and the materials released incrementally into the duodenum for digestion. It may take up to four hours for food from a complete meal to clear the stomach. 2) Chemical digestion - pepsin begins the process of protein digestion cleaving large polypeptides into shorter chains . 3) Mechanical digestion - the churning action of the muscularis causes liquefaction and mixing of the contents to produce acid chyme. 4) Some absorption - water, electrolytes, monosaccharides, and fat soluble molecules including alcohol are all absorbed in the stomach to some degree.

Protection from the acid produced by the stomach is afforded by 1) the tight junctions of the mucosal lining cells, 2) the alkaline mucus secreted by the mucous neck cells and surface mucous cells, and by 3) constant exfoliation of lining cells and their replacement by mitosis. On average the stomach lining has a 3 day turnover. Acid does occasionally make its way into the esophagus causing a burning sensation of the esophageal lining, formerly called "heartburn" and now called acid reflux disease. Peptic ulcer (see Disorders) is the name given to damage to lining cells due to stomach acid. The greatest proportion of peptic ulcers actually occur in the duodenum. A bacterium, Helicobacter pylori, has been associated with many ulcers and treatment has often focused on this bacteria. Other causative agents such as increased histamine secretion may reflect the relationship of ulcers to stress.

Control of processes in the stomach: The stomach, like the rest of the GI tract, receives input from the autonomic nervous system. Positive stimuli come from the parasympathetic division through the vagus nerve. This stimulates normal secretion and motility of the stomach. Control occurs in several phases: the cephalic phase stimulates secretion in anticipation of eating to prepare the stomach for reception of food. The secretions from cephalic stimulation are watery and contain little enzyme or acid. The gastric phase of control begins with a direct response to the contact of food in the stomach and is due to stimulation of pressoreceptors in the stomach lining which result in ACh and histamine release triggered by the vagus nerve. The secretion and motility which result begin to churn and liquefy the chyme and build up pressure in the stomach. Chyme surges forward as a result of muscle contraction but is blocked from entering the duodenum by the pyloric sphincter. A phenomenon called retropulsion occurs in which the chyme surges backward only to be pushed forward once again into the pylorus. The presence of this acid chyme in the pylorus causes the release of a hormone called gastrin into the bloodstream. Gastrin has a positive feedback effect on the motility and acid secretion of the stomach. This causes more churning, more pressure, and eventually some chyme enters the duodenum. There the intestinal phase of stomach control occurs. At first this involves more gastrin secretion from duodenal cells which acts as a "go" signal to enhance the stomach action already occurring. But as more acid chyme enters the duodenum the decreasing pH inhibits gastrin secretion and causes the release of negative or "stop" signals from the duodenum. These take the form of chemicals called enterogastrones which include GIP (gastric inhibitory peptide). GIP inhibits stomach secretion and motility and allows time for the digestive process to proceed in the duodenum before it receives more chyme. The enterogastric reflex also reduces motility and forcefully closes the pyloric sphincter. Eventually as the chyme is removed, the pH increases and gastrin and the "go" signal resumes and the process occurs all over again. This series of "go" and "stop" signals continues until stomach emptying is complete.

Summary of the events in gastric function: 1) Signals from vagus nerve begin gastric secretion in cephalic phase. 2) Presence of  food triggers release of pepsinogen and H+ in gastric phase. 3) Muscle contraction churns and liquefies chyme and builds pressure toward pyloric sphincter. 4) Gastrin is released into the blood by cells in the pylorus. Gastrin reinforces the other stimuli and acts as a positive feedback mechanism for secretion and motility. 5) The intestinal phase begins when acid chyme enters the duodenum. First more gastrin secretion causes more acid secretion and motility in the stomach. 6) Low pH inhibits gastrin secretion and causes the release of enterogastrones such as GIP into the blood, and causes the enterogastric reflex. These events stop stomach emptying and allow time for digestion in the duodenum before gastrin release again stimulates the stomach.

The duodenum is the site of most digestive enzyme release. Intensive digestion begins here. The duodenum is the first 10" of the small intestine, and receives secretions from the pancreas, from the intestinal mucosal cells, and from the liver and gallbladder. Secretions from the pancreas and bile from the gallbladder enter the duodenum through the hepatopancreatic ampulla and the sphincter of Oddi. These lie where the pancreatic duct and common bile duct join before entering the duodenum. The presence of fatty chyme in the duodenum causes release of the hormone CCK into the bloodstream. CCK is one of the enterogastrones and its main function, besides inhibiting the stomach, is to stimulate the release of enzymes by the pancreas, and the contraction of the gallbladder to release bile.  It also stimulates the liver to produce bile. Consumption of excess fat results in excessive bile production by the liver, and this can lead to the formation of gallstones from precipitation of the bile salts.   The acid in the chyme stimulates the release of secretin which causes the pancreas to release bicarbonate which neutralizes the acidity.

Summary of secretions into the duodenum and their actions: Bile - produced in the liver and stored in the gallbladder, released in response to CCK . Bile salts (salts of cholic acid) act to emulsify fats, i.e. to split them so that they can mix with water and be acted on by lipase. Pancreatic juice: Lipase - splits fats into glycerol and fatty acids. Trypsin, and chymotrypsin - protease enzymes which break polypeptides into dipeptides. Carboxypeptidase - splits dipeptide into amino acids. Bicarbonate - neutralizes acid. Amylase - splits polysaccharides into shorter chains and disaccharides. Intestinal enzymes (brush border enzymes): Aminopeptidase and carboxypeptidase - split dipeptides into amino acids. Sucrase, lactase, maltase - break disaccharides into monosaccharides. Enterokinase - activates trypsinogen to produce trypsin. Trypsin then activates the precursors of chymotrypsin and carboxypeptidase. Other carbohydrases: dextrinase and glucoamylase. These are of minor importance.

Structure and functions of the small intestine: (See Figure 24.21) the small intestine stretches nearly 20 feet, including the duodenum, jejunum and ileum. The surface area is increased by circular folds (the plicae circularis), finger-like villi, and the presence of microvilli (brush border) on the cell surfaces. At the base of the villi are the intestinal crypts, also called the intestinal glands because they are the source of the secretory cells of the mucosa. These cells are constantly renewed by mitosis and push up along the villi until they exfoliate from the surface. They cycle with about a 5-day turnover. Intestinal enzymes are released from the surface of the mucosal cells by exocytosis. These enzymes are called brush border enzymes because they cling to the microvilli. The villi possess a lamina propria beneath the epithelial lining which contains both blood and lymph capillaries for the absorption of materials. The muscularis mucosae contracts to move the villi and increase their exposure to the contents of the lumen. The three portions of the small intestine differ in subtle ways - the duodenum is the only portion with Brunner's glands in its submucosa which produce an alkaline mucus. The ileum has Peyer's Patches, concentrated lymph tissue in the submucosa. Goblet cells are progressively more abundant the further one travels along the intestine. Virtually all remaining digestion occurs in the small intestine as well as all absorption of the digestive endproducts. In addition 95% of water absorption also occurs in the small intestine. Segmentation and peristalsis propel materials through the small intestine in 4 to 6 hours.

See [Summary of Digestive-Endproducts Absorption] [Facilitated Diffusion] [Active Co-transport]

Structure and functions of the colon (large intestine): the colon is much shorter in length while larger in diameter than the small intestine. The longitudinal muscle of the colon is arranged into three distinct bands, the taenia coli, which cause the colon to buckle producing the haustra. These are pouches which increase the surface area of the colon for absorption of water and electrolytes. The colon also has deep clefts which increase its surface area. The first part of the colon is a blind pouch called the cecum. The ileum enters the cecum at the ileocecal sphincter (valve). Attached to the cecum is the vermiform (wormlike) appendix, a vestigial remnant of the larger cecum seen in other mammals. The appendix has a concentration of lymph tissue and is filled with lymphocytes, but its removal has not been demonstrated to have any negative effect on the immune system. The cecum leads in sequence to the ascending colon, then the transverse colon, the descending colon, and the sigmoid colon before entering the rectum. The rectum possesses skeletal muscle which functions during the defecation reflex.

The colon absorbs the remaining water and produces the feces. The process takes about 12 hours. Liquid chyle enters the colon through the ileocecal sphincter, whose pursed lips protrude into the cecum to help prevent backflow of chyme under pressure. This valve relaxes only when peristalsis arrives from the ileum. Muscle movements in the colon consist of: 1) minor peristaltic waves, 2) haustral churning, and 3) mass peristalsis. Haustral churning is produced by segmentation contractions which serve to mix the contents to enhance absorption. Mass peristalsis consists of large movements which occur at intervals, usually associated with meals. These movements are often initiated by the gastrocolic reflex (or gastroileal or duodenocolic reflexes) which stimulates the colon in response to food entering the stomach. This reflex is especially active after fasting and when the food is hot or cold. It causes mass peristalsis in about 15 minutes which continues for about 30 minutes. These movements cause the chyme to move in several large steps through the colon, stopping at each step to be further concentrated and converted into feces. The chyme turns from a liquid into a slush and then into firm feces. In the process some vitamins such as vitamin K and certain B vitamins are produced and absorbed, along with water and electrolytes.

The defecation reflex: As a result of the mass movements described above, pressure is exerted on the rectum and on the internal anal sphincter, which is smooth muscle, resulting in its involuntary relaxation. Afferent impulses are sent to the brain indicating the need to defecate. The external sphincter is voluntary muscle and is controlled by the voluntary nervous system. This sphincter is relaxed along with contraction of the rectal and abdominal muscles in the defecation reflex.

Disorders of the GI Tract:   [Also see Digestive Pathology] Peptic ulcer (def) has been found to be accompanied by a bacterium, Helicobacter pylori, but may also be related to stress or other factors [See Article]. Diverticulosis (def) and diverticulitis (def)  [discussion] Colon cancer [discussion] Gallbladder disease and gallstones  |  shock wave lithotripsy  |  laser lithotripsy

Revised: April 12, 2002