Ch 18 Physiology of the Digestive System

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