The Digestive System

1. The Digestive System

2. 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

3. Digestive Processes • Ingestion – occurs in the mouth • Propulsion – movement of food • Peristalsis – major means of propulsion • Mechanicaldigestion – 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

4. 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

5. 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

6. Digestive Tract Membranes • Peritoneum – a serous membrane • Parietal peritoneum – lines the body wall • Visceral peritoneum – surrounds digestive organs • Peritoneal cavity – a fluid filled space

7. 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

8. 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

9. Mesenteries • Lesser omentum attaches to lesser curvature of stomach Figure 22.10b

10. 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

11. 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

12. 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

13. 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

14. 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

15. 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

16. 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

17. 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

18. 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

19. 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

20. 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

21. 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

22. The Duodenum • Receives digestive enzymes and bile • Main pancreatic duct and common bile duct enter duodenum • Sphincters control entry of bile and pancreatic juices

23. 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

24. 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

25. 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

26. 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

27. 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

28. 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

29. 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

30. 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

31. 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

32. 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

33. 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