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: 1. Motility 5. Storage and Elimination 2. Secretion 6. Immune Barrier 3. Digestion 4. Absorption

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

5 18.2 From Mouth to Stomach Mouth Mastication = Mechanical breakdown of food Bolus is mixed with saliva composition of saliva? Digestion starts in mouth with... Deglutition

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

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

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

9 18.3 SMALL 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 pacemaker cells 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 1. Tonic Contractions sustained (where?) 2. Phasic Contractions Peristaltic contractions Progressive waves along segments of longitudinal layer forward propulsion Circular layer contracts behind bolus

14 18.4 LARGE INTESTINE Subdivided into Importance of normal microbiota. Commensal or mutualistic? Fluid and electrolyte absorption Fig 18.16

15 Absorption of H 2 O and Na + Mostly in the small intestine, some in colon Enterocytes and colonocytes use various mechanisms for Na + absorption ( Also SGLT etc. H 2 O follows

16 Absorption Review Most nutrient absorption takes place in? Fats absorbed into Everything else absorbed into Alcohol & aspirin across gastric epithelium Additional: H 2 O, ions & some vitamins absorbed in Mechanisms analogous to renal absorption

mitosis of hepatocytes Capillaries called Liver damage due

17 18.5 LIVER, 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: 1. Bile salts function? 2. Bilirubin (bile pigment), breakdown produt of heme conjugated bilirubin is water soluble

20 Bile salts are amphoteric molecules Other Liver Functions: 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: ce lls (Pancreatic Islets) make and. Exocrine: Digestive Enzymes (zymogens) and bicarbonate CFTR

22 Formation and Secretion of Bicarbonate Fig 18.28

23 18.7 DIGESTION & ABSORPTION OF FOOD Fig 18.1 Digestion of food through hydrolysis reactions

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 Fig COH Hydrolysis

27 Colon Cancer 2 nd most 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 HCl per day ph of stomach? Alkaline tide in blood Fig 21-5

32 Fat Digestion ~ 90% of dietary fat:, rest? Fig 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 H 2 O soluble vitamins absorbed by mediated transport Special case: vitamin B 12 (cobalamin) Mineral absorption by active transport

35 Summary of Dig Sys Processes Organs Involved Digestion of polymers ( ) into monomers through hydrolysis reactions Absorption of monomers

Energy, Chemical Reactions and Enzymes

Phosphorylation Hydrolysis Energy, Chemical Reactions and Enzymes Chapter 2 (selections) What is Energy? Energy is the capacity to do work Potential Energy Kinetic Energy Chemical Bond Energy Like a rechargeable