4 major processes of digestive system: Ch 21: Digestion Anatomy of digestive system (review!) 4 major processes of digestive system: Motility Secretion Digestion Absorption Fort Mackinac Mackinac Is, MI

GI Anatomy Fig 21-1

Motility Forward movement of food Mechanical mixing Secretion Digestion Absorption Motility 2 purposes: Forward movement of food Mechanical mixing GI smooth muscles contract spontaneously Pacemaker cells, (Interstitial Cells of Cajal), connected by gap junctions, generate slow wave potentials APs spread throughout longitudinal muscles (gap junctions)  wave of contraction Like cardiac muscle, Ca2+ can regulate contraction strength

Different Patterns of Contraction Tonic Contractions Sustained contraction, usually in the stomach Phasic Contractions Peristaltic contractions progressive waves moving along segments of longitudinal layer  forward propulsion circular layer contracts proximal to bolus Especially esophagus Segmental contractions alternate contraction & relaxation lead to mixing A side effect of narcotics Fig 21-4

Secretion Motility Secretion Digestion Absorption 9 L of fluid pass through the GIT (only 2 L from food & drink)  Secretion and Reabsorption important Ions and water; similar to renal mechanisms Saliva Hydrochloric Acid (Parietal Cells) Bicarbonate (enzyme necessary ?) Enzymes (zymogens) Mucus (Goblet cells) Bile (bile salts – function?) Net Fluid Balance in GI system 1/2 from accessory glands 1/2 from digestive epithelial cells Fig 21-5

Regulation of GIT p 689 ANS Emotional (cephalic reflexes) Parasympathetic (R & R) Sympathetic Emotional (cephalic reflexes) E.g., smell of food ENS (Enteric Nervous System) Self-contained (intrinsic) GI peptides can have regulatory role as hormones or paracrines E.g., Gastrin, CCK Table 21-1

Digestion Overview Bile emulsifies fats Motility Secretion Digestion Absorption Digestion Overview Mechanical breakdown and mixing aid enzymatic breakdown Chewing Tonic contractions, esp. stomach Enzymatic breakdown converts macromolecules into absorbable units Bile emulsifies fats Optimal pH of enzymes indicates location of activity

Absorption Overview Most nutrient absorption takes place in ? Motility Secretion Digestion Absorption Absorption Overview Most nutrient absorption takes place in ? Fats absorbed into lacteals Everything else absorbed into portal vein Alcohol & aspirin across gastric epithelium Additional: H2O, ions & some vitamins absorbed in ________________ Mechanisms analogous to renal absorption

CHO Digestion & Absorption ~50% of calories in average American diet Starch (polysaccharide) and sucrose (disaccharide) Cellulose (roughage) not digestible Enzymes: amylases, disaccharidases (maltase, sucrase, lactase) Absorbed only as monosaccharides (glucose, fructose) Small intestine Fig 21-14

Protein Digestion and Absorption Variable digestibility 30-60% of protein not from diet First digestion in Stomach by HCl Proteases secreted as proenzymes Pepsin(-ogen), trypsin, etc. Absorption of single a.a. and di- and tripeptides Specific receptors required for larger chains May serve as allergens (Ch 24) DDAVP

Lipid (fat) Digestion Mostly triglycerides in diet Cholesterol, Fat-soluble vitamins, others. Combination with bile salts creates an emulsion Colipase and lipase allow formation of small micelles Absorption of fat via diffusion across apical CM Chylomicrons in the cell are absorbed into lacteals See fig 21-19

Other Stuff Water soluble vitamins—mediated transport Fat soluble vits. via absorption Water,Ions and Minerals Various locations and methods, e.g, diffusion, carrier proteins Nucleic Acids

Colon Cancer 2nd largest cause of cancer deaths Cellulose (indigestible) = fiber, roughage Significance of “roughage” in diet?? behind lung cancer derived from initially benign mucosal tumors called pulyps incidence of polyp formation increases with age multistep process. one gene involved is the p53 (tumor suppressor gene)

Phases of Digestion/Absorption Cephalic Gastric Intestinal Defecation

Cephalic Gastric Intestinal Defecation Anticipation Salivation Mastication Mechanical digestion Deglutition Peristalsis in esophagus

Reflux Esophagitis = Heartburn = GERD Lower esophageal sphincter dysfunction Why reflux against gravity? See p.700

GERD, cont’d Intrapleural pressure  during inspiration  Wall of esophagus expands  Subatmospheric pressure in esophageal lumen sucks acidic stomach contents into esophagus

Cephalic Gastric Intestinal Defecation Storage Relaxation or contraction as needed. Digestion HCl (Parietal cells) Pepsin (Chief cells) Mechanical Immune Protection Ingested pathogens Respiratory mucus

Intestinal – Chyme (ingesta) enters small intestine Defecation Cephalic Gastric Intestinal – Chyme (ingesta) enters small intestine Defecation Neutralization of HCl NaHCO3 from pancreas Running Problem: Peptic Ulcer and antacids, including H2 antagonists and proton pump inhibitors Pancreatic enzymes Digest protein, CHO Bile acids Emulsion of Fat Absorption of H2O

Bacterial fermentation of CHO and proteins Cephalic Gastric Intestinal Defecation Bacterial fermentation of CHO and proteins Lactate, some vitamins are digested and/or absorbed More H2O absorption Osmotic diarrhea vs. secretory diarrhea Osmotic-solutes prevent H2O reabsorption Lactose intolerance Secretory- bacterial toxins ("flush out' pathogens) Defecation Reflex This can happen when a nonabsorbable, osmotically active substance is ingested ('osmotic diarrhea') or when electrolyte absorption is impaired ('secretory diarrhea'). Most cases of acute and chronic diarrhea are due to the latter mechanism. Secretory diarrhea can result from bacterial toxins, reduced absorptive surface area caused by disease or resection, luminal secretagogues (such as bile acids or laxatives), circulating secretagogues (such as various hormones, drugs, and poisons), and medical problems that compromise regulation of intestinal function. Fig 21-21

Lactose Intolerance Clinical focus p. 707 Lactose = glucose + galactose 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 Lactose = galactose + glucose lactose intolerance is normal, tolerance is abnormal!! undigested lactose in intestine acts osmotially to hold water with it and also candraw water form the ECF into the lumen, resulting in diarrhea. this can be linked to osmotic diuresis in diabetes, where glucose ramining in the lumen of the nephron horld water with it. In addition people complain of cramps and gas. Not all dairy products elicit same symptoms: lactose mostly absent from hard cheeses and reduced in cottage cheese ad ice cream. Yogurt with acitve bacterial cultures may be tolerated because bacteria in yogurt metabolize lactose.