Digestive System  |  Digestive Chart

• 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 end products 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 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 . 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. The serosa is continuous with the mesenteries which connect portions of the alimentary canal together

• 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.

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.

• 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.

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.

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 acid in the chyme stimulates the release of secretin which causes the pancreas to release bicarbonate which neutralizes the acidity.

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.

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.

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.

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.

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.

Nutrition

Manufacture -

• blood proteins - albumen, clotting proteins
• urea - nitrogenous waste from amino acid metabolism
• bile - excretory for the bile pigments, emulsification of fats by bile salts

Storage -

• glycogen - carbohydrate fuel
• iron - as hemosiderin and ferritin
• fat soluble vitamins A, D, E, K

Detoxification -

• alcohol
• drugs and medicines
• environmental toxins

Protein metabolism -

• transamination - removing the amine from one amino acid and using it to produce a different amino acid. The body can produce all but the essential amino acids; these must be included in the diet.
• deamination - removal of the amine group in order to catabolize the remaining keto acid. The amine group enters the blood as urea which is excreted through the kidneys.

Glycemic Regulation - the management of blood glucose.

• glycogenesis - the conversion of glucose into glycogen.
• glycogenolysis - the breakdown of glycogen into glucose.
• gluconeogenesis - the manufacture of glucose from non carbohydrate sources, mostly protein.

The liver is organized into lobes and lobules. Each lobule is served by a branch from the portal vein, the hepatic artery and a bile duct. Blood from the artery and vein mixes in sinusoids passing through the lobule. Hepatocytes line the sinusoids and withdraw digestive endproducts, toxins, etc. from the blood. Into the blood the put urea, glucose, and other substances. Into the bile ductule they put bile to be taken to the gallbladder and common bile duct. Kupffer cells are found inside the sinusoids to phagocytize debris, rbc, and pathogens.

• Absorptive Phase - in this phase digestive endproducts are being absorbed from the gut and being sent to storage. The processes occurring during this phase are:

1) glycogenesis - glucose from the blood plasma is moved into the liver for storage as glycogen. The hormone which governs this is insulin.

2) protein manufacture - amino acids absorbed from the blood are transaminated and made into proteins.

3) Fat synthesis and transport into the fat reserves of the body. Insulin enhances this as well.

• Post-absorptive Phase - in this phase glucose is moved back into the blood to supply what is used in metabolism.

1) glycogenolysis - the first source of glucose is the breakdown of glycogen. The primary hormone for this is glucagon. Epinephrine also causes release of glucose into the blood when the sympathetic nervous system is activated.

2) lipolysis - fat is broken down into glycerol and fatty acids. Glycerol is used to make glucose or in glycolysis. Fatty acids can be catabolized by many cells, especially aerobic muscle fibers. This is said to be "glucose sparing" because it leaves glucose available for those cells, e.g. neurons, which rely on glucose exclusively. Glucagon and epinephrine also trigger lipolysis. Lipolysis begins when glycogen reserves fall to about 1/3 of maximum.

3) gluconeogenesis - Amino acids are made into glucose under two conditions: a) when they are in abundance as in a high protein, low carbohydrate diet, an action mediated by glucagon; and b) when other fuel reserves are low or when severe stress causes release of cortisol. Cortisol causes proteins from muscles and connective tissue to be broken down into amino acids to make glucose. Fats are not made into glucose under normal circumstances (except for the glycerol, as above), but in high fat diets or when carbohydrate fuel is unavailable the body shifts to utilizing fat to make glucose.

insulin -  produced by the beta cells of the Islets of Langerhans, these cells are part of the endocrine portion of the pancreas which consists of islands surrounded by pancreatic exocrine cells. Insulin lowers blood glucose by triggering glycogenesis and its uptake by other cells for metabolism. The stimulus for insulin release is rising blood glucose during the absorptive phase which acts directly on the beta cells. In Type I (insulin dependent) diabetes mellitus the pancreas does not secrete enough insulin and insulin injections or oral administration are used to compensate. The amount of insulin taken must match the amount of carbohydrates consumed.

glucagon -  Is produced by the alpha cells of the Islets of Langerhans. Glucagon release is triggered by lowering blood glucose levels which occurs during the post-absorptive phase when glucose is withdrawn from the blood for metabolism. Glucagon causes glycogenolysis and lipolysis to yield more glucose and fatty acids for fuel. As blood glucose rises it will stimulate the release of insulin by the beta cells. This is a confusing yet important response. Insulin is necessary for uptake of the glucose for cellular metabolism. And by this mechanisms of two antagonistic hormones glucose levels can be precisely controlled under normal circumstances. We will more thoroughly discuss hyper- and hypoglycemia and diabetes mellitus at a later time.

epinephrine -  Epinephrine is released by the adrenal medulla into the blood and directly from sympathetic stimulation when the sympathetic nervous system is activated. It causes glycogenolysis and lipolysis which releases fuel for "fight or flight", exercise, and other stressors. NOTE: glycogenolysis in muscles releases glucose for muscle contraction, and does not directly contribute to blood glucose. However this effect is also "glucose sparing" in that plasma glucose is then available for other uses.

cortisol (cortisone) - Cortisone is released from the adrenal cortex during periods of extreme physical stress. Called a glucocorticoid, this hormone makes fuel available for metabolism, repair and replacement of tissue. This fuel comes mostly from the breakdown of proteins in muscle and connective tissue. Cortisol also has anti-immune and anti-inflammatory effects which are utilized clinically when injury occurs and to suppress the immune response. Administered over a period of time this also results in tissue damage from the hormone's catabolic effects.