Ex Nutr c5-digestion1 Gastrointestinal tract Mouth Esophagus Esophagus sphincter Stomach 3 parts:...

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Transcript of Ex Nutr c5-digestion1 Gastrointestinal tract Mouth Esophagus Esophagus sphincter Stomach 3 parts:...

  • Gastrointestinal tractMouthEsophagusEsophagus sphincterStomach3 parts: corpus (body), antrum, fundusPyloric sphincter (pylorus)Storage, mixing, regulation of emptyingNormally ~1.5L, up to 6L

  • Small intestineDuodenum, jejunum, ileumIntestinal mucosa contains folds of KerkringVilliBrush border consisting of ~600 microvilliIncrease surface area ~600 foldsVilli consists of a layer of epithelial cellsWater, water-soluble particles, electrolytes diffuse/transport into blood vessels hepatic portal vein liverWater-insoluble particles lymphatic vessels large veins

  • Mobility and transit timeFood spend 1-3 days in GI tract: transit timeTransit time in small intestine: 3-10 hrMobility: movement of food of GI tractSmall intestine contains 2 layers of smooth muscle: longitudinal and circularperistalsis

  • Gastrointestinal tractGall bladder: store, concentrate, release bileBile: produced by liver, contain electrolytes, bile salts, cholesterol, lecithin, bilirubinStore 30-60 ml, release up to 1200 ml per dayPancreasSecrete digestive enzymes, HCO3-Ileocecal valveLarge intestineAscending colon, transverse colon, descending colon, rectum, anal canalAbsorption of water and electrolytes, storage of feces

  • Regulation of GI tractSympathetic and parasympathetic nervesEndocrine glandsParacrine glands/cellsGastrin, secretin, cholecystokinin

  • Daily secretion of intestinal juicesSaliva: 1000 mlGastric secretions: 1500 mlPancreatic secretion: 1000 mlBile: 1000 mlSmall intestine: 2000 mlLarge intestine: 200 mlTotal: 6700 mlMostly reabsorbed

  • Carbohydrate digestionSaliva contains a-amylaseAlso contains lysozymes and IgA, protect from invading bacteriaCHO digestion slow down in stomachInactivation of amylase by low pH30-40% CHO digested, predominantly to oligosaccharidesPancreas secrete a-amylase to intestineAlso sucrase, lactase, maltaseBacteria in large intestine ferment cellulose (fiber) H2, CO2, volatile fatty acids (C2, C4), methane

  • Digestion of carbohydrate

  • Lipid digestionLingual lipase, slow, 10-30% TG digestionPancreatic lipase in intestineCombine with bile , form micelles (disklike) (emulsification)Absorption through villiBile salts emulsify lipids into small droplets, make hydrolysis by pancreatic lipase easierXenical

  • Digestion of fat

  • Various lipoproteins

  • Protein digestionStomach: pH ~2.0Activate pepsin (a protease) by HClDenature food proteinsIntestinePancreatic proteases secreted as inactive precursors: trypsinogen, endopeptidases, carboxypeptidases, enteropeptidasesActivated by trypsin Digest polypeptide into tripeptides, dipeptides, single amino acids

  • Digestion of protein

  • Absorption of carbohydratesMonosaccharides absorbed by carrier-mediated transport in epithelial cellSodium cotransporter, combine with Na-K-ATPase pumpSodium-independent facilitated-diffusion transporter with specificity for fructose (GLUT 5)GLUT 2 in contraluminal side

  • Absorption of CHO

  • Absorption of fatty acidsMonoacylglycerol, FA incorporated into micelles, transported to villiDiffusion through epithelial membraneIn epithelial cell, FA re-esterified into TG, form chylomicrons , transported through lymphatic systemShort- and medium-chain FA diffuse through contraluminal membrane into portal veinbile salts reabsorbed

  • Absorption of amino acidsAA, dipeptides, tripeptides absorbed by sodium-dependent active transportAt least 7 different brush border-specific transport proteins for different AACapacity of small intestine to absorb AA is very large,
  • Absorption of water99% water absorption in small intestine, by simple diffusionWater flow toward compartment with lower water concentration (higher solute concentration)Osmole: number of solute particlesmOsm/kg (osmolality), mOsm/L (osmolarity)~290 mOsm/L for most body fluidsIf ingested high osmolarity, water move into gut lumen

  • Absorption of vitaminsMostly in small intestine via diffusionDigestion free vitamins from coenzymesFat-soluble vitamins absorbed along with FAIncorporated into chylomicronsTransported through lymphatic systemWater-soluble vitaminsNot retained to any great extent in bodyAdditional vit K, B12, thiamine, riboflavin can be formed by bacteria in colon

  • Absorption of mineralsMinerals not very well absorbed in human intestineAbsorption rate, retention rate depend on intake, storage, and requirement

  • Regulation of gastric emptyingGastric motility and secretion mostly automaticStomach signals: usually positive feedbackNervous signals from stretching and extension of stomach wall, relax pyloric sphincterRelease of gastrinIntestinal signals: usually negative feedbackDuodenum contains receptors for pH, distension of duodenum, osmolarity

  • Factors for gastric emptyingFluid volume in stomach, exponentialNot affected by exercise up to 80% VO2maxShort- or long-term high intensity exercise reduce gastric emptyingHigh osmolarity reduce gastric emptyingNot important in 200-400 mOsm/L, typical for most sport drinksHigh energy density reduce gastric emptyingTemperature of food usually no effectPsychological stress reduce gastric emptying