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Digestive System - Basic Divisions • Alimentary Canal • Accessory Organs Basic Digestive Processes Wave of contraction Wall of GI tract Lumen Relaxation Bolus Ingestion Propulsion Basic Digestive Processes Mechanical Digestion Chemical Digestion Basic Digestive Processes Lumen Microvilli Epithelium Basement membrane Lamina propria Lymphatic capillary Capillary Most nutrients are absorbed into the blood capillaries Large lipids and lipid-soluble vitamins are absorbed into the lymph Absorption Defecation Oral Cavity • Extends from oral orifice to oropharynx • Site of ingestion, mechanical digestion, chemical digestion, and propulsion (deglutition). Intrinsic Salivary Glands Extrinsic Salivary Glands Parotid Gland Submandibular Gland Sublingual Gland Composition of Saliva • 97-99% water • Electrolytes • Salivary amylase • IgA • Lysozyme • Mucin Functions of Saliva • Cleanses • Dissolves and moistens Secretion of Saliva Sight/smell/thought/ taste of food Stimulates salivary nuclei in pons Parasympathetic signals to salivary glands Salivation Deglutition Esophagus • Allows for food to pass from pharynx to stomach. • Runs posterior to trachea and heart. • Goes thru diaphragm Oropharynx Laryngopharynx Esophagus Esophageal hiatus Diaphragm Stomach Esophageal Peristalsis Esophageal Histology Mucosa Submucosa Muscularis Circular layer Longitudinal layer Adventitia Lumen Stomach • Enlarged segment of digestive tract . • Links esophagus to SI • Major functions = food storage • Other functions: • production of intrinsic factor • immune defense • mechanical and chemical digestion Surface Cells and Gastric Pits Simple columnar epithelium Gastric pit Surface mucous cell (secretes alkaline fluid containing mucin) Mucous neck cell (secretes acidic fluid containing mucin) Parietal cell (secretes intrinsic factor and hydrochloric acid) Gastric gland Chief cell (secretes pepsinogen and gastric lipase) G-cell (enteroendocrine cells that secrete gastrin into the blood) Gastric Glands Simple columnar epithelium Gastric pit Surface mucous cell (secretes alkaline fluid containing mucin) Mucous neck cell (secretes acidic fluid containing mucin) Parietal cell (secretes intrinsic factor and hydrochloric acid) Gastric gland Chief cell (secretes pepsinogen and gastric lipase) G-cell (enteroendocrine cells that secrete gastrin into the blood) HCl and Pepsinogen • Pepsin = protease; active form of pepsinogen Pepsinogen • Why not secrete pepsin? Parietal cell • Note the feedback happening here. • What other function does the HCl perform? Chief cell Enteroendocrine cell HCl Pepsin Parietal Cells and Intrinsic Factor Gastric Hormones • Gastrin • released when stomach begins to stretch or due to increased vagal activity. • stimulates stomach to increase gastric juice secretion • Ghrelin • released by the empty stomach • acts on hypothalamus to increase appetite/hunger Proteins HCl Pepsinogen (inactive) Denatured proteins Oligopeptides HCl Pepsin (active) HCl Mucu s Intrinsic factor H+ Cl Mucus Mucin – Surface mucous cell Mucous neck cell Gastric gland Chief cell G-cell Gastrin Capillary Parietal cell Stomach - Danger • What about gastric juice is dangerous to the stomach itself? • How does the stomach protect itself? Gastric Muscularis Externa • 3 layers instead of 2 – Inner oblique – Middle circular – Outer longitudinal • Extra layer allows for… Gastric Motility 1 2 Contractions of smooth muscle in stomach wall mix bolus with gastric secretions to form chyme. Bolus Peristaltic wave results in pressure gradient that moves stomach contents toward the pylorus region. Pyloric sphincter Gastric secretions Pylorus region Chyme Peristaltic wave Gastric Motility 3 Pressure gradient increases force in pylorus against pyloric sphincter. 4 Pyloric sphincter opens, and a small volume of chyme enters the duodenum. 5 Pyloric sphincter closes, and retropulsion occurs. Pyloric sphincter Duodenum Chyme Retropulsion Stomach – Absorption • No nutrient absorption. • Alcohol and some drugs. Release of Gastric Juice – Cephalic Phase Release of Gastric Juice – Gastric Phase Stomach Distention Long Reflex Short Reflex Increased gastric juice secretion Increased gastrin release Peptides, caffeine Inhibition of Gastric Juice Release Release of CCK and secretin Duodenum Distention Low gastric pH Stress, anxiety Increased sympathetic release of NE Decreased gastric juice secretion What must happen to chyme as it enters the small intestine: - in terms of pH? - in terms of nutrients? What organs play a role in modifying chyme? Small Intestine Liver Gallbladder Pancreas Intestinal juice Bile Pancreatic juice Flow of Bile and Pancreatic Juice Gallbladder & Liver Pancreas Common Bile Duct Main Pancreatic Duct Hepatopancreatic Ampulla Stomach Duodenum Jejunum Ileum Surface Area of the Small Intestine • Large! (30 m2) • Maximizes enzyme secretion and nutrient absorption • 3 structural features create the large surface area Plicae Circulares • Big permanent folds of the mucosa and submucosa. • Surface area and speed bumps. Villi • • • • Fingerlike mucosal extensions Blood capillaries and lacteals Simple columnar absorptive cells Goblet cells. Microvilli • Plasma membrane extensions • Brush border • Digestive enzymes Intestinal Glands (Crypts of Lieberkuhn) • Mucosal invaginations with endocrine cells • Secrete secretin & cholecystokinin (CCK) Intestinal Submucosa • Dense connective tissue • Plumbing and wiring • Duodenum also has alkaline mucous glands • Ileum also has lymphatic nodules Peyer’s patches Duodenum Ileum Liver • Functions: – Carbohydrate metabolism – Protein metabolism – Lipid metabolism – Production of bile – Synthesis of plasma proteins – Removal of drugs, toxins etc. from the bloodstream – Phagocytosis of old/damaged RBCs – Storage of vitamins and minerals. Liver • Main digestive output is bile • Bile is sent to the gallbladder and the duodenum via ducts: – – – – R&L Hepatic Common Hepatic Common bile Cystic. Blood Supply to the Liver Hepatic lobules Right and left hepatic veins Hepatic artery Hepatic portal vein From spleen Vena cava Central vein From small and large intestine Liver Histology • Covered by a CT capsule with inward extending septa that divide the liver into hexagonal lobules. Liver Lobules Hepatic sinusoid Hepatic lobule Bile canaliculi Central vein Liver Lobule • 2 blood vessels at each corner. • Portal arteriole brings O2-rich from the hepatic artery. blood • Portal venule brings blood from abdominal viscera via the hepatic vein portal • Both empty into sinusoidal capillaries which flow past hepatocytes towards the central vein. • Central veins take blood to the hepatic vein. As blood travels thru a liver sinusoid, its: • Nutrient content will: • Oxygen content will: • Carbon dioxide content will: • Toxin content will: • Pathogen content will: • Plasma protein content will: Bile Canaliculi • Small ducts carrying bile made by hepatocytes out of the lobules and towards the hepatic ducts Bile • Produced by the liver • Watery solution with bile salts and bilirubin • Amphipathic bile salts emulsify fats Gallbladder • Located under the right lobe of the liver • Stores and concentrates bile Gallbladder - Histology • Rugae • Simple columnar epithelium with microvilli. • Thick smooth muscle muscularis. • Serosa Flow of Bile into and out of the Gallbladder Fatty chyme enters the duodenum Duodenal endocrine cells secrete CCK CCK Causes GB contraction Causes HP sphincter relaxation Inhibits gastric activity Pancreas • Deep to stomach. Stretched from spleen to duodenum • Produces alkaline fluid, digestive enzymes, and glucose-regulating hormones 99% of Pancreas = Pancreatic Acini Acinar cell (secretes amylase, lipase, proteases, and nucleases) Pancreatic acinus Duct cell (secretes HCO3–) Pancreatic Duct System Regulation of Pancreatic Secretion Chyme enters the duodenum Duodenal endocrine cells secrete CCK CCK Causes acinar cell enzyme secretion Inhibits gastric activity Causes HP sphincter relaxation Regulation of Pancreatic Secretion Chyme enters the duodenum Duodenal endocrine cells secrete secretin Secretin Causes duct cell fluid secretion Inhibits gastric activity Causes HP sphincter relaxation 1% of Pancreas = Pancreatic Islets Pancreatic acini Pancreatic islet Pancreatic Islets = Islets of Langerhans Alpha cell (secretes glucagon) Beta cell (secretes insulin) Arteriole Venule • Insulin • Decreases blood [glucose] by stimulating uptake of glucose and glycogenesis by liver and muscle cells • Increases lipogenesis and decreases lipolysis in adipocytes • Glucagon • Increases blood [glucose] by stimulating liver to perform glycogenolysis Horrible Breakfasts Large Intestine • Extends from ileocecal junction to anus. • Converts chyme into feces. • Absorbs water. • Expels feces. Large Intestine Histology Large Intestine Histology • Note the abundant goblet cells and the intestinal glands. • How do they relate to the main function of the colon? Bacterial Flora • Trillions. • Breakdown of indigestibles. • Produce vitamin K, vitamin B12, riboflavin (vitamin B2) and thiamine (vitamin B1). • How do the colonic bacteria get there? Large Intestine Motility - Haustral Contractions • Slow local movements due to haustral distention Large Intestine Motility - Mass Movements • Strong, long waves of peristaltic contraction Gastrocolic reflex Defecation Reflex Rectoanal Angle • Which is better? Feces • 100 g per day. • 75% water and 25% solids. • Contain: – – – – Dead bacteria Shed epithelial cells Fats, proteins, plant fibers Bile pigments.
