Digestion
- 1. Digestion
- 2. Digestive processes occurring in the mouth, pharynx, and esophagus Mastication is the mechanical breakdown of food (chewing). Deglutition is the transport of food from the mouth to the stomach (swallowing).
- 3. The buccal phase of swallowing occurs in the mouth, and involves the coordinated movements of the tongue and palate to force food into the oropharynx.
- 4. The pharyngeal-esophageal phase of swallowing occurs after the buccal phase, and includes blockage of the mouth by the tongue, rising of the soft palate and the epiglottis.
- 6. Digestive processes occurring in the stomach Pepsin is the primary enzyme secreted by the stomach. Intrinsic factor is required for intestinal absorption of vitamin B12, which stimulates the production of erythrocytes.
- 7. Phases of gastric secretion The cephalic (reflex) phase The gastric phase The intestinal phase
- 10. The enterogastric reflex inhibits the vagal nuclei in the medulla, inhibits local reflexes, and activates sympathetic fibers that cause the pyloric sphincter to tighten and prevent further food entry into the small intestine.
- 11. Gastric motility and emptying Receptive relaxation occurs in response to food entry, and causes the smooth muscles of the stomach to relax. Adaptive relaxation also occurs in response to food entry, but is initiated by stretch receptors in the wall of the stomach and involves nitric oxide.
- 12. Plasticity is the intrinsic ability of visceral smooth muscle to exhibit the stress- relaxation response, which is to stretch without increasing tension. The basic electrical rhythm refers to the rate of the stomach’s peristaltic waves, which is approximately three/minute, and is induced by spontaneous pacemaker cells.
- 15. Emesis (vomiting) is stomach emptying through the mouth due to stretching, or the presence of irritants. The emetic center of the medulla initiates the contraction of skeletal muscles of the abdominal wall and diaphragm.
- 16. Hormone Site of Production Stimulus Target Organs Activity Gastrin Stomach mucosa Food Stomach Causes gastric glands to increase secretory activity Stimulates gastric emptying Small intestine Stimulates contraction of intestinal muscle Ileocecal valve Relaxes ileocecal valve Large intestine Stimulates mass movements Serotonin Stomach mucosa Food Stomach Causes contraction of stomach muscle Histamine Stomach mucosa Food Stomach Activates parietal cells to release HCL Somatostatin Stomach mucosa & Food Stomach Inhibits gastric secretion, Duodenal mucosa motility, and emptying Pancreas Inhibits secretion Small intestine Inhibits GI blood flow Gallbladder Inhibits contractin & bile release
- 17. Hormone Site of Production Stimulus Target Organs Activity Secretin Duodenal mucosa Acidic Stomach Inhibits gastric gland secretion chyme and motility Pancreas Increases output of pancreatic juice rich in bicarbonate ions Liver/pancreas Increases bile output Cholecystokinin Duodenal mucosa Fatty Liver Augments secretin's actions chyme Pancreas Increases output of enzyme rich pancreatic juice Gallbladder Stimulates organ to contract and expel stored bile Hepatopancreatic Relaxes to allow entry of bile sphincter and pancreatic juice into duodenum
- 18. Intestinal juice: Composition and control Intestinal juice has a pH similar to that of blood, is isotonic with blood plasma, and is composed of water, and mucus, but is enzyme poor.
- 19. Regulation of bile release into the small intestine Gallstones can obstruct the flow of bile from the gall bladder, and form as a result of the crystallization of cholesterol due to either too much cholesterol or too few bile salts.
- 22. Digestive processes occurring in the small intestine The migrating mobility complex is the pattern of peristaltic activity initiated in the duodenum, whose waves sweep along the intestine 10-70 cm before dying out. Enhanced activity of the stomach initiates the gastroileal reflex, which is a long reflex that enhances the force of segmentation in the ileum.
- 23. Digestive processes occurring in the large intestine Haustral contractions are slow segmenting movements that occur every 30 minutes, stimulated by distension of its muscles. Mass movements are powerful contractile waves that move over large areas of the colon three or four times daily, and force the contents toward the rectum.
- 24. The gastrocolic reflex is the colon’s equivalent to the gastroileal reflex in the small intestine, and is responsible for initiating mass movements. Diverticulitis is a result of lack of bulk in the colon, causing small herniations of the colon called diverticula which become inflamed.
- 25. Defecation reflex causes emptying of the rectum due to stretching of the rectal wall as a result of mass movements. Diarrhea is watery stools, which results from any condition that rushes food residue through the large intestine before that organ has had sufficient time to absorb the remaining water.
- 26. When food remains in the colon for extended periods, too much water is absorbed and the stool becomes hard and difficult to pass, resulting in a condition called constipation.
- 28. Mechanism of chemical digestion: Enzymatic hydrolysis Chemical digestion is a catabolic process in which large food molecules are broken down to monomers small enough to be absorbed by the GI tract lining. The enzymatic breakdown of any type of food molecule is called hydrolysis, due to the addition of a water molecule to each molecular bond broken.
- 30. Chemical digestion of specific food groups The monomers of carbohydrates are called monosaccharides. Salivary amylase splits starch into smaller fragments of two to eight linked monosaccharides.
- 31. Carbohydrates that escape being broken down by salivary amylase are acted on by pancreatic amylase. Dextrinase and glucoamylase are brush border enzymes that act on polysaccharides composed of three simple sugars.
- 32. Maltase, sucrase, and lactase are brush border enzymes that hydrolyze maltose, sucrose, and lactose respectively into their constituent monosaccharides.
- 33. Pepsin begins protein digestion in the stomach, when it is converted in an acidic environment, from pepsinogen. Rennin is a stomach enzyme that coagulates milk protein, but is not produced in adults.
- 35. Trypsin and chymotrypsin are pancreatic enzymes secreted into the duodenum that cleave protein fragments into smaller peptides.
- 36. Carboxypeptidase, secreted by both the intestinal brush border and the pancreas, splits off one amino acid at a time from the end of the polypeptide chain with the carboxyl group. Aminopeptidase and dipeptidase are brush border enzymes that liberate the final amino acid products.
- 38. Lipases are fat-digesting enzymes secreted by the pancreas, that hydrolyze triglyceride molecules, yielding free fatty acids and monoglycerides.
- 40. Nucleic acids are hydrolyzed into their nucleotide monomers by pancreatic nucleases. Nucleotides are broken down by nucleosidases and phosphatases, which are intestinal brush-border enzymes.
- 41. Absorption
- 42. Absorption of specific nutrients Micelles are collections of fatty elements clustered together with bile salts in such a way that the polar ends of the molecules face the water and the nonpolar portions from the micelle core.
- 44. Chylomicrons are composed of triglycerides combined with small amounts of phospholipids and a cholesterol, and coated with proteins to form water-soluble lipoprotein droplets. The triglycerides of chylomicrons are hydrolyzed to free fatty acids and glycerol by lipoprotein lipase in the bloodstream.
- 45. Iron binds with a protein called ferritin after it is transported into mucosal cells, which is like a storehouse for iron. Iron binds with a protein called transferrin in the blood, which transport it in the circulation.
- 46. Calcium absorption is regulated by the active form of vitamin D, which acts as a cofactor to facilitate calcium absorption.
- 47. Malabsorption of nutrients Malabsorption can result from anything that interferes with the delivery of bile or pancreatic juice to the small intestine, as well as factors that damage the intestinal mucosa.