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The Digestive System Overview of the Digestive System • Organs are divided into two groups – Alimentary canal • Mouth, pharynx, and esophagus • Stomach, small intestine, and large intestine – Accessory digestive organs • Teeth and tongue • Gallbladder, salivary glands, liver, and pancreas Digestive Processes • • Ingestion – occurs in the mouth Propulsion – movement of food – Peristalsis – major means of propulsion • Mechanical digestion – prepares food for chemical digestion – Chewing, churning of food in stomach, segmentation • Chemical digestion – complex molecules broken down to chemical components – Mouth – Stomach – Small intestine • • Absorption – transport of digested nutrients Defecation – elimination of indigestible substances as feces Digestive Tract Smooth Muscle Movement • Peristalsis – Major means of propulsion – Adjacent segments of the alimentary canal relax and contract • Segmentation – Rhythmic local contractions of the intestine – Mixes food with digestive juices Figure 22.3a Histological Organization of Digestive Tract • Mucosa lines digestive tract (mucous epithelium) – – – • • • Epithelium lines the lumen, moistened by secretions of mucosal glands Lamina propria – loose areolar tissue Muscularis mucosa – layer of smooth that creates folds called rugae Submucosa - layer of dense irregular connective tissue, vascularized (blood and lymphatic), innervated – nerve plexus, submucosal glands Muscularis externa - smooth muscle arranged in circular and longitudinal layers Adventitia or Serosa - serous membrane made of areolar connective tissue with collagen and elastic fibers, covers most of the muscularis externa Digestive Tract Membranes • Peritoneum – a serous membrane – Parietal peritoneum – lines the body wall – Visceral peritoneum – surrounds digestive organs • Peritoneal cavity – a fluid filled space Digestive Tract Membranes • Mesentery – a double layer of peritoneum – Holds organs in place – Sites of fat storage – Provides a route for circulatory vessels and nerves Mesenteries • Greater omentum – a “fatty apron” of peritoneum – Attaches the greater curvature of the stomach to the dorsal body wall – Covers the transverse colon and a large part of the small intestine Figure 22.10c Mesenteries • Lesser omentum attaches to lesser curvature of stomach Figure 22.10b Oral cavity • Lined by oral mucosa = stratified squamous but no serosa • Roof of cavity = hard and soft palates • Uvula guards opening to pharynx • Floor of cavity = tongue – Mechanical processing – Assistance in chewing and swallowing – Sensory analysis by touch, temperature, and taste receptors The Oral Cavity • Its functions include: – – – – Analysis of material before swallowing Mechanical processing by the teeth, tongue, and palatal surfaces Lubrication Limited digestion Figure 24.6a, b Salivary glands (three pairs) • Parotid, sublingual, and submandibular • Produce saliva - watery solution includes electrolytes, buffers, glycoproteins, antibodies, enzymes • Functions include: lubrication, moistening, and dissolving, Initiation of digestion of complex carbohydrates Teeth • Function in mastication of bolus • Contain three layers – Enamel covering crown – Dentin forms basic structure – Root - periodontal ligaments hold teeth in alveoli • 20 primary teeth AKA deciduous teeth • 32 teeth of secondary dentition The Pharynx • • • • Oropharynx and laryngopharynx Common passageway for food, liquids, and air Lined with stratified squamous epithelium, no serosa External muscle layer – Consists of superior, middle, and inferior pharyngeal constrictors – Pharyngeal constrictor muscles assist in swallowing The Esophagus • Muscular tube - begins as a continuation of the pharynx • Carries solids and liquids from the pharynx to the stomach – Passes through esophageal hiatus in diaphragm • Joins the stomach inferior to the diaphragm – Cardiac sphincter – closes lumen to prevent stomach acid from entering esophagus • The wall of the esophagus contains mucosal (stratified squamous), submucosal, and muscularis layers The Esophagus – – – – Epithelium is stratified squamous epithelium When empty – mucosa and submucosa in longitudinal folds Mucous glands – primarily compound tubuloalveolar glands Muscularis externa • Skeletal muscle first third of length – Adventitia – most external layer The Stomach • Site where food is churned into chyme, Mechanical breakdown of food • Breaking of chemical bonds via acids and enzymes, decretion of pepsin and HCl begins protein digestion • Bulk storage of undigested food, food remains in stomach approximately 4 hours Anatomy Of The Stomach • • • • • Cardia – superior, medial portion Fundus – portion superior to stomach-esophageal junction Body – area between the fundus and the curve of the J Pylorus – antrum and pyloric canal adjacent to the duodenum Rugae - ridges and folds in relaxed stomach Microscopic Anatomy of the Stomach • Muscularis has three layers – Circular and longitudinal layers and oblique layer • Epithelium is simple columnar epithelium • Mucosa dotted with gastric pits holds gastric glands Microscopic Anatomy of the Stomach • Gastric glands of fundus and body – Mucous neck cells - secrete a special mucus – Parietal (oxyntic) cells - secrete hydrochloric acid and gastric intrinsic factor – Chief (zymogenic) cells - secrete pepsinogen • Glands produce 1500ml juice per day Small intestine • Longest portion of the alimentary canal • Site of most enzymatic digestion and absorption – Secretions and buffers provided by pancreas, liver, gall bladder • Three subdivisions: – Duodenum – Jejunum – Ileum • Ileocecal sphincter transition between small and large intestine The Duodenum • Receives digestive enzymes and bile • Main pancreatic duct and common bile duct enter duodenum – Sphincters control entry of bile and pancreatic juices The Small Intestine – Microscopic Anatomy • Modifications for absorption – Circular folds (plicae circulares) - transverse ridges of mucosa and submucosa – Villi - finger-like projections of the mucosa, covered with simple columnar epithelium – Microvilli - further increase surface area for absorption – Lacteals - terminal lymphatic in villus Histology of the Intestinal Wall • • • • Absorptive cells - uptake digested nutrients Goblet cells - secrete mucus that lubricates chyme Enteroendocrine cells - secrete hormones Intestinal crypts - epithelial cells secrete intestinal juice Large Intestine • Digested residue contains few nutrients • Small amount of digestion by bacteria • Main functions – – – – absorb water and electrolytes compact material into feces Absorb vitamins produced by bacteria Store fecal matter prior to defecation Gross Anatomy of Large Intestine • Four areas of the colon – – – – Ascending Transverse Descending Sigmoid • Special features of large intestine – Teniae coli - thickening of longitudinal muscularis – Haustra - puckering created by teniae coli – Cecum - blind pouch, beginning of large intestine – Vermiform appendix Contains lymphoid tissue The Rectum • Rectum - descends along the inferior half of the sacrum • Anal Canal - the last subdivision of the large intestine – Lined with stratified squamous epithelium – Last portion of the digestive tract – Terminates at the anal canal – Internal and external anal sphincters – Defecation reflex triggered by distention of rectal walls Microscopic Anatomy of Large Intestine • Villi are absent • Contains numerous goblet cells • Intestinal crypts – simple tubular glands • Lined with simple columnar epithelial tissue – Epithelium changes at anal canal, becomes stratified squamous epithelium The pancreas • Pancreatic duct penetrates duodenal wall • Endocrine functions - insulin and glucagons • Exocrine functions - majority of pancreatic secretions, pancreatic juice secreted into small intestine – – – – Carbohydrases Lipases Nucleases Proteolytic enzymes The Pancreas • Exocrine function – Acinar cells make, store, and secrete pancreatic enzymes – Enzymes are activated in the duodenum • Endocrine function – Produces insulin and glucagon – Regulates blood sugar The liver • Performs metabolic and hematological regulation and produces bile • Four Lobes: left, right, caudate, quadrate • Histological organization – – – – Lobules containing single-cell thick plates of hepatocytes Lobules empty into bile ducts, bile ducts merge to left/right hepatic ducts L/R hepatic ducts merge to form common hepatic duct Common hepatic meets cystic duct to form common bile duct Liver lobule - basic functional unit • Hepatocytes form irregular plates arranged in spoke-like fashion • Bile canaliculi carry bile to bile ductules • Bile ductules lead to bile ducts in portal areas The Gallbladder • Hollow, pear-shaped organ • Stores, modifies and concentrates bile • Bile – bile salts help buffer acids and acts as emulsifier for lipid digestion Figure 24.21a, b