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Ch 18 Physiology of the Digestive System

Ch 18 Physiology of the Digestive System. SLOs. List the functions of the digestive system Distinguish and describe the different patterns of motility observed in the GI tract.

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Ch 18 Physiology of the Digestive System

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  1. Ch 18 Physiology of the Digestive System

  2. SLOs • List the functions of the digestive system • Distinguish and describe the different patterns of motility observed in the GI tract. • Name and explain the various secretions associated with the GI tract, namely saliva, acid, bicarbonate, mucus, digestive enzymes, bile. • Explain how pH can be used to predict the location where a particular digestive enzyme might be most active. • Outline the digestion and absorption mechanisms and locations for carbohydrates, proteins, fats, vitamins, minerals, ions and water. • Explain what lactose intolerance and colon cancer are and how they come about

  3. 18.1 Introduction Review your Anatomy! • 6 major processes of digestive system: • Motility • Secretion • Digestion • Absorption Storage and Elimination Immune Barrier

  4. Layers of the GI Tract • Mucosa vs. Serosa ? • Muscle layers ? • Extrinsic vs. intrinsic regulation • Hormones Regulation of GI Tract Fig 18.3

  5. 18.2 From Mouth to Stomach Mouth • Mastication= _______________ Mechanical breakdown of food • Bolusis mixed with salivacomposition of saliva? • Digestion starts in mouth with .... • Deglutition

  6. Esphagus Epithelial layer – why? Peristalsis Lower (gastroesphageal) sphincter GERD? Fig 18.4

  7. Stomach Epithelial layer – why? Churning of food to mix with gastric secretions: • Acid • Digestion? Chyme enters .... Clinical App: Gastric Bypass SurgeryVitamin ____ Deficiency. Why? Fig 18.6

  8. Digestion and Absorption in the Stomach Most absorption takes place in _____________ Absorbed in stomach are 1) 2) Peptic ulcers: Gastric or duodenal Activation of pepsin: Fig 18.8

  9. 18.3Small Intestine Starts at the pyloric sphincter: 3 sections • Plicae circulares • Villi • Microvilli Brushborder enzymes

  10. Clinical App: Lactose Intolerance • Lactose = ? • Lactase only found in juvenile mammals • Adaptive (dominant) mutation in populations with dairy-based cultures • Lactose intolerance in • 95% of Native Americans, • 90% of Asian Americans • 70% of African Americans • 50% of Mexican Americans • Well tolerated are hard cheeses and yoghurt, otherwise: • Osmotic diarrhea

  11. Intestinal Contractions and Motility 2 purposes: • Forward movement of food: Peristalsis • Mechanical mixing: Segmentation GI smooth muscles contract spontaneously • Endogenous pacemakercells generate slow wave potentials • APs spread throughout longitudinal muscles (gap junctions)  wave of contraction Fig 18.8

  12. Segmental contractions = Segmentation alternate contraction & relaxation  mixing Compare to Fig 18.13

  13. Different Patterns of Contraction • Tonic Contractions  sustained (where?) • Phasic Contractions Peristaltic contractions Progressive waves along segments of longitudinal layer  forward propulsion Circular layer contracts behind bolus

  14. 18.4Large Intestine • Subdivided into • Importance of normal microbiota. Commensal or mutualistic? • Fluid and electrolyte absorption Fig 18.16

  15. Absorption of H2O and Na+ • Mostly in the small intestine, some in colon • Enterocytes and colonocytes use various mechanisms for Na+ absorption ( • Also SGLT etc. • H2O follows

  16. Absorption Review • Most nutrient absorption takes place in ? • Fats absorbed into ______ • Everything else absorbed into ______ • Alcohol & aspirin across gastric epithelium • Additional: H2O, ions & some vitamins absorbed in ________________ • Mechanisms analogous to renal absorption

  17. 18.5Liver, Gallbladder, and Pancreas Liver is largest abdominal organ Amazing regenerative abilities due to mitosis of hepatocytes Capillaries called _______ Liver damage due to alcohol or viral hepatitis  Liver cirrhosis Hepatic portal system

  18. Flow of blood and bile in a liver lobule Fig 18.20

  19. Major Liver Functions Bile production and secretion Bile stored in ……. • Key components: • Bile salts – function? • Bilirubin(bile pigment), breakdown produt of heme  conjugated bilirubin is water soluble

  20. Other Liver Functions: Bile salts are amphoteric molecules  • Production of plasma proteins. Which? • Secretion of glucose, triglycerides, and ketone bodies, etc. See also Table 18.3

  21. Pancreas Endocrine and exocrine functions Endocrine: ___________________cells (Pancreatic Islets) make _________and _________. Exocrine: Digestive Enzymes (zymogens) and bicarbonate CFTR

  22. Formation and Secretion of Bicarbonate Fig 18.28

  23. 18.7Digestion & Absorption of Food Digestion of food through hydrolysis reactions Fig 18.1

  24. Digestion Overview • Mechanical breakdown aids enzymatic breakdown • Enzymatic breakdown converts macromolecules into absorbable units • Optimal pH of enzymes indicates location of activity

  25. COH Digestion & Absorption ~50% of calories in average American diet; major sources? COH digestion mainly in 2 places Enzymes: __________, _______________ (maltase, sucrase, lactase) Absorbed only as monosaccharides(glucose, fructose etc.)

  26. COH Hydrolysis Fig 18.32

  27. Colon Cancer • 2ndmost common cause of cancer deaths • Cellulose (indigestible) = fiber, roughage • Significance of “roughage” in diet??

  28. Protein Digestion 30 - 60% of protein found in GI lumen is from dead sloughing cells Protein digestion starts in ? Plant proteins least digestible Endopeptidases (Proteases) vs. Exopeptidases Absorption in form of ?

  29. Endo- and Exopeptidases Aminopetidase Carboxipetidase Fig 18.33

  30. Peptide Absorption Absorption of proteins/ peptides can lead to food allergies, (e.g.: gluten) Drug companies develop indigestible protein/peptide drugs (e.g.: modified vasopressin)

  31. Secretion • 9 L of fluid pass through (only 2 L from food & drink)  Reabsorption important • Ions and water (see renal mechanisms). Remember CFTR • Acid • 1 to 3 L of HClper day • pH of stomach? • Alkaline tide in blood Fig 21-5

  32. Fat Digestion Fig 18.34 ~ 90% of dietary fat: ____________, rest ? Fats form large clumps in aqueous chyme (hard to digest) Lipase aided by 2 non-enzyme secretions: • Bile acids or -salts→ emulsify • Colipase→ allows for lipase access to bile salt coated droplets Lipase activity: Triglycerides Monoglyceride + 2 free FA

  33. Fat Digestion & Absorption cont. Absorption via ____________ In epithelial cells: Re- synthesis into TG in sER Chylomicrons Lacteals Adipose/muscle tissue Chylomicron remnants to liver VLDLs, LDLs, HDLs

  34. Vitamin & Mineral Absorption • Fat soluble vitamins absorbed with fat in small intesting • H2O soluble vitamins absorbed by mediated transport • Special case: vitamin B12(cobalamin) • Mineral absorption by active transport

  35. Organs Involved Summary of Dig Sys Processes Digestionof polymers (________________________________) into monomers through hydrolysis reactions Absorptionof monomers The End

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