Slide 1Slide 2Slide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Slide 16Slide 17Slide 18Slide 19Slide 20Slide 21Slide 22Slide 23Slide 24Slide 25Slide 26Slide 27Slide 28Slide 29Slide 30Slide 31The Digestive System:4. SecretionFigure 21.3Mass Balance in the Digestive SystemSecretions include:• Water -paracellular• Ions -membrane channels -paracellular• Proteins -secreted by cellsParacellular transport of water in the GI tractin response to a solute concentration gradientFigure 5.2Figure 21.3Secretions include:• Water -paracellular• Ions -membrane channels -paracellular• Proteins -secreted by cellsMass Balance in the Digestive SystemExocrine Gland SecretionSerous secretions: watery solutions containing ions and enzymesMucous secretions: mucus, sticky solutionsof glycoproteins and proteoglycans (goblet cells)Acinus/Acini :rounded terminal end/s of exocrine gland (acinar cells)Salivary SecretionLumenInterstitial fluidExtrinsic Salivary Glands-sublingual-submandibular-parotidIntrinsic Salivary GlandsThe “Salivon”:Composition of saliva: WaterInorganic Ions (Na+, K+, Cl-, HCO3-, F-)Organic compounds (enzymes, mucins)Functions of saliva: LubricationWaterMucusTasteWater : Food molecules must be dissolved for recognition by taste receptorsProtectionMucusCl-, HCO3-Antibacterial enzymes: Lysozyme and IgAF-Digestiona-amylaseLingual lipase(neither one is essential for digestion)Control of salivary secretionClassical Conditioning ExperimentIvan Pavlov, 1901Control of salivary secretion-Reduction of salivary secretions during fight-or-flight (acute stress) response -> controlled by sympathtic nervous systemFigure 21.13Water secretion in the GI tract:Cl- secretion by salivary gland, intestinal and colonic crypt cellsOnly mucus is secreted -important for motilityEsophageal SecretionFunctions of the stomach:StorageMixingSecretes gastric juice (exocrine)Secretes chemical messengers (endocrine/paracrine)Gastric SecretionModified Figure 21.9aopening of gastric glandendocrine/paracrine secretionsexocrine secretionsComponents of gastric juice:• Hydrochloric acid (HCl)• Pepsinogen - proenzyme that is converted to pepsin (zymogen)• Gastric Lipase• Intrinsic factor - vitamin B12-binding protein Mucus/Bicarbonate (HCO3- )Functions of gastric juice:DigestionHCl - denatures proteins- breaks chemical bonds in food- activates pepsin from pepsinogenPepsinogen proteaseGastric Lipase fat digestionProtectionHCl kills bacteriamucus/ HCO3- protects stomach wall from HCl, pepsin, and mechanical traumaIntestinal absorption of Vitamin B12Intrinsic factor complexes with Vitamin B12Pepsinogeninhibited catalytic domainH+uninhibited catalytic domainautolytic activationActive PepsinActivation of pepsin by HClComponents of gastric juice:• Hydrochloric acid (HCl)• Pepsinogen - proenzyme that is converted to pepsin (zymogen)• Gastric Lipase• Intrinsic factor - vitamin B12-binding protein Mucus/Bicarbonate (HCO3- )Functions of gastric juice:DigestionHCl - denatures proteins- breaks chemical bonds in food- activates pepsin from pepsinogenPepsin proteaseGastric Lipase fat digestionProtectionHCl kills bacteriamucus/ HCO3- protects stomach wall from HCl, pepsin, and mechanical traumaIntestinal absorption of Vitamin B12Intrinsic factor complexes with Vitamin B12Figure 21.9Secretory cells of the Gastric MucosaFigure 21.9cHydrochloric acid secretion by parietal cells• H+ is actively pumped into lumen against electrochemical gradient• Cl- follows through open chloride channels along the electrochemical gradient• Bicarbonate is absorbed into blood -> “alkaline tide” after mealCA= carbonic anhydraseEndocrineReleases insulin and glucagon (facilitate glucose storage and release)ExocrineSecretes digestive enzymesSecretes HCO3-Acid can damage the duodenumAcid inactivates digestive enzymesPancreatic SecretionPancreatic ductSphincter of OddiDigestive Enzymes Secreted by Pancreas:Secreted in active form:a-AmylaseLipaseCarboxyl ester lipaseDNaseFatsCarbohydrates?Secreted as zymogens (inactive precursors):TrypsinogensChymotrypsinogensProcarboxypeptidase A and BProelastase(Proteases) ProteinProcolipaseFatsFigure 21.14cBicarbonate secretion in pancreas and duodenumNa+ and H20 reach lumen through paracellular transport following electrochemical and osmotic gradients.Figure 21.15aLiver SecretionFigure 21.12Figure 21.15bFigure 21.15cHepatocytes secret bile into bile canaliculi -> bile ductule -> common hepatic duct->gall bladder ->common bile duct -> release into duodenum-> reabsorption of bile components with nutrients -> return to liver through hepatic portal vein-> reabsorption of bile salts by hepatocytesFigure 21.15dThe GallbladderFunctions:1. Stores bile between meals(Normally closed)2. Concentrates bileJaundiceSecretion in the Small IntestineSecreted components: water, ionsmucushormonesFunction of secreted components:Protection: -HCO3- neutralizes acid-mucus protects wall from digestionControl: -secreted hormones regulate motility, secretions of pancreas and liver, gastric emptying and gastric secretionsalkaline mucus :- protects wall from acid released by bacteria-eases passage of fecesSecretion in the Large