GE 345: Week 5
Small Intestine
| Intro | Mouth | Stomach | Sm Int | Colon | |
The presence of chyme stretches the small intestine, which triggers localized concentric contractions at intervals, causing segmentation of the intestine, and chopping the chyme to mix the food particles with secretions.
Peristaltic waves also propel food through the small intestine, with net movement of about 1 cm/min, taking 3-5 hours for chyme to pass from pylorus to the ileocecal valve, where it passes into the colon. Controlled by gastroenteric reflex, and secreted hormones, including gastrin, cholecystokinin (cck), insulin and serotonin, which enhance intestinal motility, and secretin and glucagon, which inhibit motility. Peristaltic rush (powerful contractions which can travel long distances in minutes) can be caused by irritants in the chyme.
Digestion and Absorption
The duodenum receives secretions from both the pancreas and liver. The pancreas secretes digestive enzymes for proteins, carbohydrates and fats, and bicarbonate ions to neutralize stomach acid as the chyme enters the duodenum.
Most carbohydrate hydrolysis occurs in the upper part of the small intestine under the influence of pancreatic amylase. Pancreatic amylase hydrolyzes starches, glycogen and other carbohydrates except cellulose into disaccharides and trisaccharides. The enzymes lactase, sucrase, maltase and alpha-dextrinase, split disaccharides and small carbohydrates into monosaccharides glucose, galactose, and fructose, which are absorbed into the portal blood. Functionally, all carbohydrates are absorbed by active transport as monosaccharides. It is believed the carrier protein responsible for monosaccharide transport wont work unless a sodium ion is also present.
Proteolytic pancreatic enzymes: trypsin (the most abundant), chymotrypsin, carboxypeptidase and proelastase split proteins into small polypeptides; Epithelial cells of the small intestinal villi contain enzymes (peptidases) which split peptides intotripeptides, dipeptides, and amino acids, which are then absorbed into the epithelial cells where the remaining tri and dipeptides are digested into aa's. Proteolytic enzymes are in inactive forms until reaching the digestive tract to prevent digestion of the pancreas. Protein absorption is also controlled by a sodium co-transport mechanism similar to that for carbohydrate. If properly masticated and not eaten in too large quantity at one time, 98% of protein is absorbed as amino acids, and 2% is excreted into the feces.
95-99% of all fat digestion occurs in the small intestine. Fats are first emulsified by bile salts secreted by the liver, which break the fat globules into small sizes by decreasing surface tension on the fat globule. Pancreatic lipase then hydrolyzes neutral fat into fatty acid and monoglycerides. Cholesterol esterase causes hydrolysis of cholesterol esters, and phospholipase splits fatty acids from phospholipids. Bile salts bind with the monoglyerides and free fatty acids to form micelles, and carry them to the brush borders of the epithelial cells, where they are absorbed via diffusion, leaving the bile acid micelles in the chyme, where they pick up more fatty acids. 97% of fatty acids are absorbed with sufficient micelles. In the absence of the bile acids, only 40-50% are absorbed. Fatty acids and monoglycerides are then formed into new triglycerides in the smooth ER, although some are further digested into glycerol and fatty acids by intracellular lipase then combined with newly synthesized glycerol to form entirely new triglycerides. The triglycerides aggregate first inthe ER, then the GA into globules along with cholesterol, and phospholipids. They form globules, which are transported into the lymph where they eventually are transported to the veins.
Bile also serves as a means for exretion of waste products from the blood, including bilirubin (end-product of hemoglobin destruction) and excess cholesterol synthesized by liver cells.
About 8-9 liters (1.5 l ingested, 7 l secreted) of fluid must be absorbed each day. Most absorption (about 7 l) occurs in the small intestine. The absorptive surface of the intestinal mucosa has folds which increase the surface area about 3 times. Villi project about 1 from the mucosal surface as well. Epithelial cells on the surface of the villi, consisting of microvilli, which increase surface area another 20-fold.
Water is abosrbed via diffusion, passing through pores quite readily, and often follows transported substances into the circulation. Most ions are actively transported through the membrane, although in separate processes.