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769852281 Digestion I. Overview A. Organs 1. Alimentary canal a. Function i. Digests ii. Absorbs b. Organs i. Mouth ii. Pharynx iii. Esophagus iv. Stomach v. Small intestine vi. Large intestine 2. Accessory digestive a. Organs i. Teeth ii. Tongue iii. Gallbladder b. Glands i. Salivary ii. Liver iii. Pancreas II. Digestive Process A. Essential activities 1. Ingestion a. Taking food into the digestive system via the mouth 2. Propulsion a. Movement of food through the digestive tract b. Swallowing i. Voluntary c. Peristalsis i. Involuntary 3. Mechanical digestion a. Chewing b. Mixing i. Saliva c. Churning d. Segmentation i. Local constriction of the small intestine 4. Chemical digestion a. Mouth through small intestine (see below) 5. Absorption a. Movement of digested end products from lumen of the GI tract into blood and lymph 6. Defecation a. Elimination of undigested materials III. Histology of the Alimentary Canal A. Four basic layers -1- 769852281 1. Mucosa a. Innermost layer b. Functions i. Secretion ii. Absorption iii. Protection c. Sub-layers i. Lining epithelium ii. Lamina propria iii. Muscularis mucosae d. Epithelium i. Simple columnar with goblet cells e. Lamina propria i. Loose areolar connective tissue ii. Capillary beds iii. Lymph nodules f. Muscularis mucosae i. Thin layer of smooth muscle 2. Submucosa a. Dense connective tissue i. Blood vessels ii. Lymphatic vessels b. Elastic fibers 3. Muscularis externa a. Segmentation and peristalsis b. Inner circular and outer longitudinal layer of smooth muscle c. Sphincters i. Thickened areas of smooth muscle 4. Serosa a. Areolar connective tissue covered with mesothelium i. Single layer of squamous epithelium IV. Functional Anatomy -2- 769852281 A. Mouth (oral cavity or buccal cavity) 1. Boundaries a. Lips b. Cheeks c. Tongue d. Palate 2. Palate a. Hard palate i. Underlain by bone ii. Rigid b. Soft palate i. Formed from muscle B. Tongue 1. Bundles of skeletal muscle 2. Function a. Mix food with saliva i. Bolus b. Position bolus for swallowing C. Salivary glands -3- 769852281 1. Function of saliva a. Cleanses mouth b. Dissolves tastants c. Moistens d. Initiates chemical breakdown 2. Types a. Extrinsic i. Three pairs ii. Parotid iii. Submandibular iv. Sublingual b. Intrinsic i. Buccal glands 3. Secretory cells a. Serous cells i. Watery secretion with enzymes b. Mucous cells i. Viscous D. Teeth -4- 769852281 1. Classification a. Incisors b. Canines c. Premolars d. Molars 2. Dental formula: (2I, 1C, 2PM, 3M/2I, 1C, 2PM, 3M) X 2 = 32 E. Pharynx 1. Oropharynx to Laryngopharynx (see Respiration Lecture) F. Esophagus 1. Laryngopharynx into esophagus 2. Pierces diaphragm 3. Joins stomach a. Cardiac orifice b. Cardiac sphincter 4. Four layers a. Mucosa b. Submucosa c. Muscularis i. Skeletal and smooth ii. Distribution is regionalized d. Adventia (not serosa) i. Entirely connective tissue G. Digestive processes in mouth, pharynx and esophagus 1. Mastication a. Mechanical breakdown by teeth and tongue 2. Deglutition -5- 769852281 a. Complicated process of swallowing i. Two phases ii. Involves 22 muscles H. Stomach 1. Expansion of the GI tract 2. Sack for storage and chemical breakdown a. Food converted to chyme V. Stomach A. Gross anatomy 1. Regions a. Cardiac i. Region where food enters -6- 769852281 b. Fundus c. Body d. Pyloric i. Terminates at the pylorus ii. Connects with small intestine iii. Pyloric sphincter 2. Greater curvature a. Lateral convex surface 3. Lesser curvature a. Medial concave surface B. Microscopic anatomy 1. Four tunics 2. Lining epithelium a. Simple columnar i. Entirely goblet cells b. Gastric pits i. Gastric glands ii. Produce gastric juice 3. Secretory cells of gastric glands a. Mucous neck cells i. Produce acidic mucus b. Parietal cells -7- 769852281 i. Secrete HCl and intrinsic factor c. Chief (zymogenic) cells i. Produce pepsinogen ii. Converted initially by HCl into pepsin iii. Pepsin will catalyze conversion thereafter d. Enteroendocrine cells i. Produce hormones that regulate digestive function (see below) C. Digestive processes occurring in stomach 1. Enzymatic digestion a. Protein digestion is initiated in stomach i. Pepsin 2. Lipid soluble substances can pass through stomach mucosa a. Alcohol and aspirin 3. Production of intrinsic factor a. Required for absorption of B12 D. Regulation of gastric secretion 1. Three phases a. Cephalic b. Gastric c. Intestinal 2. Cephalic phase -8- 769852281 a. Occurs prior to food entering stomach i. Brain response to food b. Hypothalamic stimulation of the vagus nerve i. Parasympathetic enteric ganglionic neurons stimulate the stomach glands 3. Gastric phase a. Local signaling within stomach due to the presence of food i. Distension ii. Peptides iii. Low acidity b. Lead to HCl release i. Gastrin is released in response to chemical stimuli ii. Gastrin stimulates the release of HCl by parietal cells 4. Intestinal phase -9- 769852281 a. Excitatory phase i. Duodenum releases a gastrin-like substance that promotes additional activity of gastric glands b. Inhibitory phase—enterogastric reflex i. Inhibit vagal nuclei ii. Inhibit local reflexes iii. Activate sympathetic fibers that tighten pyloric sphincter iv. Cause the release of enterogastrones 5. Enterogastrones a. Secretin b. Cholecystokinin (CCK) c. Vasoactive intestinal peptide (VIP) d. Gastric inhibitory peptide (GIP) VI. Small Intestine A. Gross anatomy - 10 - 769852281 1. Pyloric sphincter to ileocecal valve 2. Subdivisions a. Duodenum b. Jejunum c. Ileum B. Microscopic anatomy 1. Expanded surface area for absorption 2. Structural modifications a. Circular folds i. Deep folds of the mucosa and submucosa b. Villi i. Fingerlike projections of the mucosa c. Microvilli (brush border) - 11 - 769852281 i. Projections of plasma membranes C. Histology 1. Simple columnar epithelium 2. Submucosa include lymphatic tissue a. Peyer’s patches D. Liver and gallbladder 1. Accessory organs associated with small intestine - 12 - 769852281 2. Liver has a role in digestion in addition to its other functions a. Bile production and export i. Emulsification of fat 3. Gallbladder a. Stores bile 4. Bile a. Only bile salts and phospolipids aid in digestion b. Other components that do not contribute to digestion (not discussed) c. Emulsify fat 5. Regulation of bile release - 13 - 769852281 a. CCK from small intestine is released into blood in response to fatty chyme entering small intestine b. CCK stimulates secretion of pancreatic juice c. CCK relaxes hepatopancreatic sphincter i. Controls entry of pancreatic juice and bile entering duodenum E. Pancreas 1. Accessory digestive organ 2. Produces digestive enzymes a. Exocrine product—pancreatic juice 3. Acini a. Secretory cells surrounding ducts 4. Composition of pancreatic juice a. Proteases - 14 - 769852281 b. Amylase c. Lipases d. Nucleases 5. Regulation of pancreatic secretion a. Local hormones i. CCK ii. Secretin b. Parasympathetic nervous system VII. Large Intestine A. Gross anatomy 1. Ileocecal valve to the anus - 15 - 769852281 2. Absorbs water from indigestible food residues 3. Unique features a. Teniae coli i. Three bands of longitudinal smooth muscle b. Haustra i. Pocketlike sacs c. Epiploic appendages i. Fat-filled pouches 4. Subdivisions a. Cecum i. Saclike ii. First segment b. Appendix i. Lymphatic ii. Dead end c. Colon i. Several regions ii. Ascending colon iii. Transverse colon iv. Descending colon v. Sigmoid colon d. Rectum i. Rectal valves ii. Internal transverse folds e. Anal canal i. Two sphincters ii. Internal and external iii. Internal is involuntary iv. External is voluntary 5. Digestive processes a. No breakdown b. Water and vitamin absorption c. Major function is the peristaltic movement of fecal material 6. Defecation - 16 - 769852281 a. Rectal walls stretch i. Defecation reflex b. Spinal-parasympathetic reflex i. Causes colon and rectum to contract ii. Anal sphincters relax VIII. Chemical Digestion A. Background 1. Hydrolysis a. Catabolic process b. Large molecules into monomers c. Enzymes into lumen of alimentary canal i. Intrinsic and accessory glands - 17 - 769852281 B. Carbohydrates (starch) 1. Carbohydrates are broken down into glucose, fructose and galactose 2. Process a. Salivary amylase b. Pancreatic amylase C. Proteins 1. Begins in stomach a. Pepsinogen to pepsin 2. Continues in small intestine a. Trypsin and chymotrypsin i. Pancreatic enzymes b. Carboxypepsidase i. Pancreatic enzymes D. Lipids - 18 - 769852281 1. Digestion occurs solely in small intestine a. Lipases from pancreas 2. Bile emulsifies fats so they are soluble a. Only increases area that enzymes can contact E. Nucleic acids 1. Pancreatic nucleosides IX. Absorption A. Active transport through the intestinal villi 1. Lipids can pass through cell membranes a. Active transpost is not requires 2. Carbohydrates a. Protein carriers i. Metabolic energy is required - 19 -