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Intestinal secretion a. Mucus: most likely serves a protective role, preventing HCl and chyme for digestion the intestinal wall. Mucus is secreted by:1- Brunner s gland, which are located within the duodenum .2- Goblet cell located along the length of the intestinal epithelium and the intestinal crypts, called the crypts of Lieberkun. b. Enzymes: capable of breaking down small peptides and disaccharides are associated with the microvilli of the epithelial cells lining the intestine. Although these enzymes are not secreted into the intestine, they are able to digest small peptides and disaccharides during the absorptive process. c. Water and electrolytes: are secreted by all the epithelial cells of the intestine. The watery secretion provides a solvent into which the products of digestion are dissolved. If excessive amounts of fluid are produced (as happens when the enterotoxin responsible for cholera stimulates massive fluid secretion), potentially life- threatening watery diarrhea can result. Digestion and absorption: a. Carbohydrates: The three major carbohydrates in the human are the disaccharides : sucrose(cane sugar ) and lactose (milk sugar) as well as the polysaccharide starches (which may be in either the straight chain from amylase, or the branched chain from amylopectin ). Cellulose, another plant polysaccharide, is present in the diet in large amounts but no enzymes in the human digestive tract can digest it, so it is excreted unused. Dietary intake of carbohydrates is 250800g/day, which represents 50%-60% of diet Digestion: b. Carbohydrates must be digested into monosaccharide before being absorbed from the GIT: c. a. Although starch digestion, by salivary amylase, begins in the mouth, almost all carbohydrate digestion occurs within the small intestine. d. b. Pancreatic amylase digests carbohydrates into variety of oligosaccharides. e. c. The oligosaccharides are digest into monosaccharides by brush border enzymes such as maltase, lactase, and sucrase. f. d. The end products of carbohydrates are fructose, glucose, and galactose.