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Chapter 15 - Digestion! Digestive System - Chapter 15 Figure 15.1! 1! Digestive System Functions! 1. Ingestion! 2. Mechanical processing! • E.g. teeth, stomach ! • Increase surface area for attack by enzymes! • Smaller pieces are easier to move! 3. Digestion! • Proteins → amino acids! • Triglycerides → glycerol + fatty acids! • Polysaccharides → monosaccharides! 2! 1! Chapter 15 - Digestion! Functions - 2! 4. Absorption! • Absorb across digestive tract epithelium, eventually into blood! 5. Secretion! • Water, acids, enzymes, buffers, ions! 6. Excretion! • Removal of wastes - defecation! 3! The Structure of the Digestive Tract "! Figure 15.2! 4! 2! Chapter 15 - Digestion! h"p://histology-‐world.com/ Layers of the GI tract" Lumen" Mucosa" 5! General Histology of G.I. Tract! Four layers modified from a general plan to carry out the function(s) of a particular region! 1. Mucosa - mucous membrane! Epithelium (in contact with food)! Type varies with location! E.g. Oral cavity - stratified squamous EPI! "Many layers of flat cells! • Resists abrasion! E.g. Small intestine - simple columnar! One layer of tall cells! • Secretion, absorption! 6! 3! Chapter 15 - Digestion! General Histology - 2! 2. Submucosa - ! "a) Loose connective tissue! Binds mucosa to muscularis (externa)! b) N.A.V.a.L! c) Submucosal plexus! "Part of autonomic N.S. - “Brain” of gut! d) Glands! 7! General Histology - 4! 3. Muscularis (externa) ! Smooth (involuntary) muscle – most of the tract! Inner circular, outer longitudinal ! "(stomach has an extra layer - why??)! Skeletal (voluntary) muscle! "Mouth, pharynx, upper part of esophagus! 4. Serosa! "Holds organs loosely in place! "Reduces friction! " "e.g. Mesentery, mesocolon, etc.! 8! 4! Chapter 15 - Digestion! Peristalsis! Peristalsis - Waves of muscular contraction! Continuously moves bolus from mouth toward anus! 1. Circular muscle contracts behind bolus, relaxes in front of bolus! 2. Longitudinal muscle contracts in front of bolus! 3. Circular muscle contracts, pushes bolus forward! ! 9! Peristalsis" " " " " Figure 15.6! 10! 5! Chapter 15 - Digestion! Tongue! Manipulates food for chewing, swallowing; functions in speech! Sensory: taste, touch, temperature)! Secretion: mucins and lingual lipase (below)! Lingual glands secrete lingual lipase! • Begins digestion of fats to fatty acids and monoglycerides! • Works over wide pH range (3.0 - 6.0)! • May continue to work in stomach for a while! 11! The Salivary Glands" " " Figure 15.4"! 12! 6! Chapter 15 - Digestion! Salivary Glands! 1. Parotid salivary glands: Anterior to ears! • 25% of saliva! • contains salivary amylase! 2. Submandibular (submaxillary) salivary gland! • Under mandible! • 70% of saliva! • Salivary amylase and mucus! 3. Sublingual salivary gland, under tongue! • 5% of saliva! • Almost entirely mucus! 13! Composition of Saliva! About 1 liter produced every day!! ! 99.4% water, rest is solutes! Contains:! • Ions - e.g. chloride (activates salivary amylase)! • Salivary amylase (begin CH2O digestion)! • Bicarbonate and phosphate buffers" • Mucus (lubricates food, helps form bolus)! 14! 7! Chapter 15 - Digestion! Teeth " " " " " Figure 15.3b! 15! Mechanical and Chemical Digestion in Mouth! Mastication = chewing! Food mixed with saliva = bolus! Enzymes in saliva! Salivary amylase! • Begins CH2O digestion! Lingual lipase (from tongue)! • Begins triglyceride (fat) digestion! 16! 8! Chapter 15 - Digestion! Esophagus! Passes through diaphragm to stomach! 1. Mucosa! • Stratified squamous epithelium! • Resists abrasion by food! 2. Submucosa! • Mucous glands - lubrication! 3. Muscularis (externa)! • Peristalsis moves bolus to stomach! 17! Esophageal “Sphincters”! Upper and lower (cardiac) “sphincters”! Are physiological sphincters, not anatomical sphincters like pyloric or ileocolic sphincters! Muscle tone in circular muscle keeps ends closed! Problem can cause acid reflux ! 18! 9! Chapter 15 - Digestion! Stomach Anatomy " " " Figure 15.7a! Fundus! Cardia! Body! Pyloric sphincter! Rugae! (folds)! 19! The Stomach Lining" " " Figure 15.7b"! No villi present! 20! 10! Chapter 15 - Digestion! Stomach Histology! 1. Mucosa - folded into gastric pits with gastric glands! Cell types:! A. Mucous cells! • Secrete alkaline mucus! B. Chief cells! • Secrete pepsinogen! ! " " "HCl and/or pepsin! Pepsinogen ------------------------->Pepsin! Pepsin begins protein digestion! 21! Stomach Histology - 2! C. Parietal cells! Secrete HCl! ! HCl functions:! • Kills bacteria! • Denatures enzymes/other proteins in food! • Helps activate pepsinogen to pepsin! ! 22! 11! Chapter 15 - Digestion! Stomach Histology - 3! 2. Submucosa! Similar to rest of G.I. Tract! 3. Muscularis externa! Has three layers of smooth muscle! Adds inner oblique to circular and longitudinal layers! Why does it make sense that the stomach has an extra smooth muscle layer?" 4. Serosa! 23! Digestion/Absorption in the Stomach! Digestion:! A. Begin limited digestion of proteins! B. Carbohydrates and lipids! • Salivary amylase and lingual lipase! No nutrient absorption because:! No nutrient transporters present!! ! Some absorption of:! Alcohol (EtOH), aspirin, lipid-soluble drugs! 24! 12! Chapter 15 - Digestion! Small Intestine - Parts! Most chemical digestion and nutrient absorption occurs here! 21 feet long in cadaver; 10 feet long in living person (Why longer in the cadaver?)! 1. Duodenum (“12” finger widths)! • Entrance = pylorus of stomach! • Glands produce alkaline mucus (Why?)! 2. Jejunum (“empty” in cadaver)! 3. Ileum (“twisted”)! • Lots of lymphoid tissue (Why?) ! • Exits into colon at ileocecal valve! 25! Small Intestine Histology! Modified to increase surface area for secretion and absorption! ! Compared to a simple tube:! 1. Circular folds" • Permanent, visible! "folds of mucosa and submucosa" • Increase luminal surface area by 3X! 26! 13! Chapter 15 - Digestion! Small Intestine Histology - 2! 2. Villi" Folds of the mucosa" Contain lacteals! Are lymphatic capillaries! ü Transport fat + protein to blood! Increase luminal surface area by another 10X! 27! The Intestinal Wall - Villi! Martini 2004! See also Figure 15.8b! 28! 14! Chapter 15 - Digestion! Small Intestine Histology - 3! 3. Microvilli" Folds of epithelial cell membrane" Fold contains cytoplasm! Form what is called the “brush border”! Contains brush border enzymes digestion! ! Increase surface area by another 20X! 29! Intestinal Surface Area - The Bottom Line! Total increase in surface area (for digestion and absorption) compared to a simple, unmodified tube is:! 3 x 10 x 20 = 600x !! ! That means that your small intestine would need to be 600 times longer to do the same amount of work. If it did not have these surface area modifications your small intestine would need to be:! 600 X 10 feet = 6,000 feet long! 30! 15! Chapter 15 - Digestion! The Pancreas " " " " Figure 15.12! 31! Pancreas! A heterocrine gland! Exocrine portion involves secretion into ducts" Secretes about 1.5 l/day of pancreatic juice into the duodenum! • Digestive enzymes! • Sodium bicarbonate (NaHCO3): high pH neutralizes acidic chyme from stomach)! Endocrine portion secretes hormones " "Pancreatic islets secrete insulin (and others)! 32! 16! Chapter 15 - Digestion! The Liver " " " " " "Figure 15.10! 33! Liver Functions! 1. Metabolic regulation! A. CH2O, Lipid and amino acid metabolism! B. Waste removal! e.g. ammonia converted to urea (less toxic)! C. Vitamin storage (fat-soluble vitamins)! D. Mineral storage (e.g. iron storage)! E. Drug/toxin inactivation (smooth ER)! 34! 17! Chapter 15 - Digestion! Liver Functions - 2! 2. Hematological regulation! A. Phagocytosis and antigen presentation! B. Synthesize plasma proteins! C. Remove circulating hormones and antibodies! ! 3. Bile synthesis! 35! Bile Functions - 1! About 800 - 1,000 ml produced per day by liver! Contains:! Water, bile salts, bile acids, cholesterol, pigments, ions! Functions:! A. Digestive! Bile emulsifies fats - Increases surface area for digestion by lipases! Is not an enzyme - does NOT digest fats! B. Excretory! Gets rid of heme portion of hemoglobin molecules from worn out RBCs! 36! 18! Chapter 15 - Digestion! The Gallbladder " " " "Figure 15.12! 37! Gallbladder! Functions:! 1. Bile storage (does NOT make bile)! 2. Bile modification! • Reabsorbs water, concentrates bile! 3. Bile release:! Presence of lipids in small intestine causes CCK release →! • Gallbladder contracts! • Bile enters small intestine ! 38! 19! Chapter 15 - Digestion! The Large Intestine "(Colon) " "Figure 15.13! Ileocecal valve! 39! Large Intestine! Functions:! 1. Reabsorption of water – formation of feces! 2. Absorption of vitamins ! "e.g. vitamin K produced by bacteria! 3. Storage of feces! Anatomy:" About 5 feet long! Ileocecal valve (sphincter) connects with small intestine! 40! 20! Chapter 15 - Digestion! Large Intestine - Anatomy 2! Ileum opens into colon at cecum via ileocecal valve! Appendix attached to colon! Rectum/Anus! Temporary storage of feces! • Internal anal sphincter! Smooth muscle = involuntary! Stretch initiates urge to defecate! • External anal sphincter! Skeletal muscle = voluntary! 41! Large Intestine - Histology! Mucosa! No villi (unlike small intestine)! Has intestinal (mucous) glands! Epithelial cells! • Absorptive cells (water)! • Mucous cells (lubricating mucus)! • Do NOT produce digestive enzymes! ! Lots of lymph nodules (why?)! 42! 21! Chapter 15 - Digestion! Large Intestine - Histology 2! Submucosa! Many lymphoid nodules! Muscularis externa! Inner circular smooth muscle layer! 3 longitudinal smooth muscle bands = taeniae coli! Contractions produce haustra (“pouches”)! Serosa! 43! Secretion and Absorption of Water" ! Small intestine:! 9000 in - 1200 ml out = 7800 ml reabsorbed! Large intestine:! 1400 in - 150 ml out = 1250 ml reabsorbed! 44! 22! Chapter 15 - Digestion! Secretion and Absorption of Water! "Food, drink " " "2000 ml! Saliva " " " "1500 ml! Gastric secretion " "1500 ml! Intestinal secretion " "2000 ml! Bile " " " "1000 ml! Pancreatic secretion " "1000 ml! Colon mucus " " " 200 ml! "" " " " "9200 ml! Lost in feces - 150 ml! Most reabsorption in small intestine: "water follows salt (osmosis)! " " 45! Summary Table 15.1! 46! 23! Chapter 15 - Digestion! Summary Table 15.3! 47! 24!
