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Digestive System Introduction 1. Ingest food 2. Transport ingested material 3. Secrete substances needed for digestion • Enzymes, etc 4. Digest material into smaller usable components 5. Absorb digested nutrients into bloodstream 6. Expel waste products from body Introduction 1. Digestive organs – gastrointestinal (GI) tract 2. Accessory organs – secrete substances into GI tract Oral Cavity • Site of mechanical and chemical digestion 1. Cheeks, lips and palate 2. Tongue • • Manipulates and mixes food Compresses food into bolus 3. Salivary glands • Produce and secrete saliva 4. Teeth • Mechanically digest food – Break into smaller pieces Permanent Teeth • 32 total – each quadrant has: – Incisors (2) – shaped like chisel, for gnawing – Canines (1) – pointed tips for puncturing and tearing – Premolars (2) – flat crowns with prominent ridges for crushing and grinding – Molars (3) – also adapted for crushing and grinding Salivary Glands • Saliva serves the following functions: 1. 2. 3. 4. 5. Moistens ingested materials Moistens, cleanses and lubricates oral structures Chemical digestion Antibacterial action Dissolves materials to stimulate taste receptors Parotid Salivary Glands • Secrete 25-30% of total saliva • Via parotid duct • Also secretes amylase – Begins chemical digestion of starch • Innervated by which CN? Submandibular and Sublingual Salivary Glands • Submandibular salivary glands – Produce the majority of saliva (60-70%) • Sublingual salivary glands – Contribute only 3-5% of total saliva • Innervated by which CN? Pharynx • Shared by respiratory and digestive systems • Which two CN innervate? Peritoneum • Serous membrane of abdominal cavity: 1. Parietal peritoneum 2. Visceral peritoneum • Organs may be: – – Intraperitoneal Retroperitoneal • Lie against posterior cavity wall Mesenteries • Folds of peritoneum – support and stabilize intraperitoneal GI tract organs – Contain blood vessels, lymphatic vessels and nerves • Include: – Greater omentum – hangs from stomach – Lesser omentum – connects stomach to liver – Mesentery proper – suspends SI from posterior wall – Mesocolon – attaches LI to posterior wall The Wall of the Abdominal GI Tract • Composed of four tunics: 1. 2. 3. 4. Mucosa Submucosa Muscularis Serosa Mucosa • Three components: 1. Simple columnar ET – Except esophagus 2. Lamina propria (CT) • • Contains lymph nodules MALT – mucosa-associated lymphatic tissue – T cells, B cells, plasma cells, and macrophages 3. Smooth muscle layer called muscularis mucosa • May have villi – Increase surface area – Contains lacteal • Lymph capillary Submucosa • Components include: 1. 2. 3. 4. Lymphatic ducts Mucin-secreting glands Blood vessels Nerves Muscularis • Two layers of smooth muscle: 1. Inner circular layer – constricts lumen and forms sphincters • Involved with peristalsis 2. Outer longitudinal layer – shortens tube • Involved with segmentation Esophagus • Tubular, muscular passageway • Passes through esophageal hiatus of diaphragm • At junction of esophagus and stomach = gastroesophageal (cardiac) sphincter – Prevents regurgitation into esophagus Stomach • Continues mechanical and chemical digestion of bolus – processed into paste-like soup called chyme • Three layers of muscle – Additional oblique layer Regions of the Stomach • Four regions: 1. 2. 3. 4. Cardiac region Fundus Body Pylorus • • Pyloric sphincter regulates movement of chyme into SI internal surface folded – rugae (gastric folds) Wall of the Stomach • Little absorption occurs in stomach • Stomach mucosa contains gastric pits Gastric Secretions • Along gastric pits are gastric glands • Mucous neck cells - Mucus • Parietal cells: • HCl - Protein breakdown • Intrinsic factor - Vit B12 absorption • Chief cells – Pepsinogen • Precursor of pepsin - enzyme for protein digestion • G cells –Gastrin • Hormone that stimulates other gastric glands Small Intestine • About 22 feet long • Finishes chemical digestion process – Carbs, proteins, and lipids • Absorbs majority of nutrients • Takes ~ 12 hrs to digest and absorb food Small Intestine • Duodenum – Brunner’s glands in submucosa secrete alkaline mucus – Contains hepatopancreatic ampulla and sphincter • Opening of ducts from liver, gallbladder, and pancreas • Jejunum – Primary region of digestion and absorption • Ileum – Contains Peyer’s patches (MALT) – Ileocecal valve regulates passage into LI – Secretions of the SI – CCK – stimulates release of bile from gall bladder – Sucrase, lactase, maltase – digest disaccharides Histology of the Small Intestine • Mucosa and submucosa have circular folds (plicae circularis) • Folds have villi • Villi have microvilli • All serve to increase surface area for absorption Large Intestine • Shorter than SI, but larger diameter • Functions: – Absorbs fluids and ions – Compacts indigestible wastes into feces – Stores feces until defecation • Intestinal flora – Over 700 species of bacteria – Produce vitamins (K, B12) – Facilitate water absorption – Digest plant matter • Creates gas! Large Intestine • Comprised of: 1. Cecum • appendix 2. Colon • • • • Ascending colon Transverse colon Descending colon Sigmoid colon 3. Rectum 4. Anal canal Rectum • Expands to store fecal material • Rectal valves ensure fecal material retained during passage of gas Anal Canal • Anal sinuses secrete mucin for lubrication during defecation • Internal and external anal sphincters control defecation – Internal – smooth muscle – External – skeletal muscle Liver • Produces bile • Detoxifies • Processes nutrients Gall Bladder • On inferior surface of liver • Collects and concentrates bile from liver • Stimulated to contract by CCK from SI after eating fatty meal Biliary Apparatus • Ducts transport bile from liver and gall bladder to duodenum Liver R. Hepatic duct Gall Bladder L. Hepatic duct Common hepatic duct Cystic duct Common bile duct Duodenum Pancreas • Located inferior to stomach in curve of duodenum • Exocrine functions involve secreting pancreatic juices into duodenum via pancreatic ducts Pancreatic Juice • Contains digestive enzymes – Trypsin – protein digestion • Many other proteases too – Lipase – lipid digestion – Amylase – starch digestion • Contains bicarbonate – Buffer – Neutralizes acidity from stomach