Ch 18 Physiology of the Digestive System SLOs

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. Name and explain the various secretions associated with the GI tract, namely saliva, acid, bicarbonate, mucus, digestive enzymes, bile. Explain how p. H 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

18. 1 Introduction Review your Anatomy! 6 major processes of digestive system: 1. Motility 2. Secretion 5. Storage and Elimination 3. Digestion 6. Immune Barrier 4. Absorption

Layers of the GI Tract Mucosa Muscle vs. Serosa ? layers ? Regulation of GI Tract Extrinsic vs. intrinsic regulation Hormones Fig 18. 3

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

Esphagus Epithelial layer – why? Peristalsis Lower (gastroesphageal) sphincter GERD? Fig 18. 4

Stomach Epithelial layer – why? Churning of food to mix with gastric secretions: –Acid –Digestion? Chyme enters. . Clinical App: Gastric Bypass Surgery Fig 18. 6

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

18. 3 SMALL INTESTINE Starts at the pyloric sphincter: 3 sections – Plicae circulares – Villi – Microvilli Brushborder enzymes

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:

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

Segmental contractions = Segmentation alternate contraction & relaxation mixing Compare to Fig 18. 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

18. 4 LARGE INTESTINE Subdivided into Importance of normal microbiota. Commensal or mutualistic? Fluid and electrolyte absorption Fig 18. 16

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

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

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

Flow of blood and bile in a liver lobule Fig 18. 20

Major Liver Functions Bile production and secretion Bile stored in ……. Ø Key components: 1. Bile salts – function? 2. Bilirubin (bile pigment), breakdown produt of heme conjugated bilirubin is water soluble

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

Pancreas Endocrine and exocrine functions – Endocrine: __________ cells (Pancreatic Islets) make _____and _____. – Exocrine: Digestive Enzymes (zymogens) and bicarbonate – CFTR

Formation and Secretion of Bicarbonate Fig 18. 28

18. 7 DIGESTION &ABSORPTION OF FOOD Fig 18. 1 Digestion of food through hydrolysis reactions

Digestion Overview Mechanical breakdown aids enzymatic breakdown Enzymatic breakdown converts macromolecules into absorbable units Optimal p. H of enzymes indicates location of activity

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. )

COH Hydrolysis Fig 18. 32

Colon Cancer 2 nd most common cause of cancer deaths Cellulose (indigestible) = fiber, roughage Significance of “roughage” in diet? ?

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 ?

Endo- and Exopeptidases Aminopetidase Carboxipetidase Fig 18. 33

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

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 – p. H of stomach? – Alkaline tide in blood Fig 21 -5

Fat Digestion ~ 90% of dietary fat: ______, rest ? Fig 18. 34 Fats form large clumps in aqueous chyme (hard to digest) Lipase aided by 2 non-enzyme secretions: Ø Bile acids or -salts → emulsify Ø Colipase → allows for lipase access to bile salt coated droplets Lipase activity: Triglycerides Monoglyceride + 2 free FA

Fat Digestion & Absorption cont. Absorption via ______ In epithelial cells: Re- synthesis into TG in s. ER Chylomicrons Lacteals Adipose/muscle tissue Chylomicron remnants to liver

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

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