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Understand the anatomy, functions, and regulation of the gastrointestinal tract. Learn about digestion, absorption, and key enzymes involved in processing carbohydrates, fats, and proteins. Explore absorption mechanisms for nutrients and water.
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Ex Nutr c5-digestion Gastrointestinal tract • Mouth • Esophagus • Esophagus sphincter • Stomach • 3 parts: corpus (body), antrum, fundus • Pyloric sphincter (pylorus) • Storage, mixing, regulation of emptying • Normally ~1.5L, up to 6L
Ex Nutr c5-digestion Small intestine • Duodenum, jejunum, ileum • Intestinal mucosa contains folds of Kerkring • Villi • Brush border consisting of ~600 microvilli • Increase surface area ~600 folds • Villi consists of a layer of epithelial cells • Water, water-soluble particles, electrolytes diffuse/transport into blood vessels hepatic portal vein liver • Water-insoluble particles lymphatic vessels large veins
Ex Nutr c5-digestion Mobility and transit time • Food spend 1-3 days in GI tract: transit time • Transit time in small intestine: 3-10 hr • Mobility: movement of food of GI tract • Small intestine contains 2 layers of smooth muscle: longitudinal and circular • peristalsis
Ex Nutr c5-digestion Gastrointestinal tract • Gall bladder: store, concentrate, release bile • Bile: produced by liver, contain electrolytes, bile salts, cholesterol, lecithin, bilirubin • Store 30-60 ml, release up to 1200 ml per day • Pancreas • Secrete digestive enzymes, HCO3- • Ileocecal valve • Large intestine • Ascending colon, transverse colon, descending colon, rectum, anal canal • Absorption of water and electrolytes, storage of feces
Ex Nutr c5-digestion Regulation of GI tract • Sympathetic and parasympathetic nerves • Endocrine glands • Paracrine glands/cells • Gastrin, secretin, cholecystokinin…
Ex Nutr c5-digestion Daily secretion of intestinal juices • Saliva: 1000 ml • Gastric secretions: 1500 ml • Pancreatic secretion: 1000 ml • Bile: 1000 ml • Small intestine: 2000 ml • Large intestine: 200 ml • Total: 6700 ml • Mostly reabsorbed
Ex Nutr c5-digestion Carbohydrate digestion • Saliva contains a-amylase • Also contains lysozymes and IgA, protect from invading bacteria • CHO digestion slow down in stomach • Inactivation of amylase by low pH • 30-40% CHO digested, predominantly to oligosaccharides • Pancreas secrete a-amylase to intestine • Also sucrase, lactase, maltase • Bacteria in large intestine ferment cellulose (fiber) • H2, CO2, volatile fatty acids (C2, C4), methane
Ex Nutr c5-digestion Digestion of carbohydrate
Ex Nutr c5-digestion Lipid digestion • Lingual lipase, slow, 10-30% TG digestion • Pancreatic lipase in intestine • Combine with bile 膽鹽, form micelles (disklike) 乳化 (emulsification) • Absorption through villi • Bile salts emulsify lipids into small droplets, make hydrolysis by pancreatic lipase easier • Xenical 羅氏纖
Ex Nutr c5-digestion Digestion of fat
Ex Nutr c5-digestion Various lipoproteins
Ex Nutr c5-digestion Protein digestion • Stomach: pH ~2.0 • Activate pepsin (a protease) by HCl • Denature food proteins • Intestine • Pancreatic proteases secreted as inactive precursors: trypsinogen, endopeptidases, carboxypeptidases, enteropeptidases • Activated by trypsin 胰蛋白脢 • Digest polypeptide into tripeptides, dipeptides, single amino acids
Ex Nutr c5-digestion Digestion of protein
Ex Nutr c5-digestion Absorption of carbohydrates • Monosaccharides absorbed by carrier-mediated transport in epithelial cell • Sodium cotransporter, combine with Na-K-ATPase pump • Sodium-independent facilitated-diffusion transporter with specificity for fructose (GLUT 5) • GLUT 2 in contraluminal side
Ex Nutr c5-digestion Absorption of CHO
Ex Nutr c5-digestion Absorption of fatty acids • Monoacylglycerol, FA incorporated into micelles, transported to villi • Diffusion through epithelial membrane • In epithelial cell, FA re-esterified into TG, form chylomicrons 乳糜微粒, transported through lymphatic system • Short- and medium-chain FA diffuse through contraluminal membrane into portal vein • bile salts reabsorbed
Ex Nutr c5-digestion Absorption of amino acids • AA, dipeptides, tripeptides absorbed by sodium-dependent active transport • At least 7 different brush border-specific transport proteins for different AA • Capacity of small intestine to absorb AA is very large, <1% ingested protein in feces
Ex Nutr c5-digestion Absorption of water • 99% water absorption in small intestine, by simple diffusion • Water flow toward compartment with lower water concentration (higher solute concentration) • Osmole: number of solute particles • mOsm/kg (osmolality), mOsm/L (osmolarity) • ~290 mOsm/L for most body fluids • If ingested high osmolarity, water move into gut lumen
Ex Nutr c5-digestion Absorption of vitamins • Mostly in small intestine via diffusion • Digestion free vitamins from coenzymes • Fat-soluble vitamins absorbed along with FA • Incorporated into chylomicrons • Transported through lymphatic system • Water-soluble vitamins • Not retained to any great extent in body • Additional vit K, B12, thiamine, riboflavin can be formed by bacteria in colon
