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Chapter 24

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  1. Chapter 24 The Digestive System

  2. Digestive System: Overview • The alimentary canal or gastrointestinal (GI) tract digests and absorbs food • Alimentary canal – mouth, pharynx, esophagus, stomach, small intestine, and large intestine • Accessory digestive organs – teeth, tongue, gallbladder, salivary glands, liver, and pancreas

  3. Digestive System: Overview Figure 24.1

  4. Digestive Process • The GI tract is a “disassembly” line • Nutrients become more available to the body in each step

  5. Digestive Process • There are six essential activities: • ingestion, propulsion, and mechanical digestion • chemical digestion, absorption, and defecation Figure 24.2

  6. Essential Activities of Digestion • Ingestion – taking food into the digestive tract • Propulsion – swallowing and peristalsis • Peristalsis – waves of contraction and relaxation of muscles in the organ walls • Mechanical digestion – chewing, mixing, and churning food

  7. Essential Activities of Digestion Figure 24.3a, b

  8. Essential Activities of Digestion • Chemical digestion – catabolic breakdown of food • Absorption – movement of nutrients from the GI tract to the blood or lymph • Defecation – elimination of indigestible solid wastes Figure 24.2 Figure 24.2

  9. Histology of the Alimentary Canal • From esophagus to the anal canal the walls of the GI tract have the same four tunics • From the lumen outward they are the mucosa, submucosa, muscularis externa, and serosa • Each tunic has a predominant tissue type and specific digestive function

  10. Histology of the Alimentary Canal Figure 24.6

  11. Mucosa • Moist epithelial layer that lines the lumen of the alimentary canal • Its three major functions are: • Secretion of mucus • Absorption of the end products of digestion • Protection against infectious disease

  12. Mucosa: Epithelial Lining • Consists of simple columnar epithelium and mucus-secreting goblet cells • The mucus secretions: • Protect digestive organs from digesting themselves • Ease food along the tract • Stomach and small intestine mucosa contain: • Enzyme-secreting cells and • Hormone-secreting cells (making them endocrine and digestive organs)

  13. Mucosa: Other Sublayers • Submucosa – dense connective tissue containing elastic fibers, blood and lymphatic vessels, lymph nodes, and nerves • Muscularis externa – responsible for segmentation and peristalsis • Serosa – the protective visceral peritoneum • Replaced by the fibrous adventitia in the esophagus • Retroperitoneal organs have both an adventitia and serosa

  14. Salivary Glands • Produce and secrete saliva that: • Cleanses the mouth • Moistens and dissolves food chemicals • Aids in bolus formation • Contains enzymes that breakdown starch • Three pairs of extrinsic glands – parotid, submandibular, and sublingual • Intrinsic salivary glands (buccal glands) – scattered throughout the oral mucosa

  15. Salivary Glands II Figure 24.9a

  16. Saliva: Source and Composition • Secreted from serous and mucous cells of salivary glands • A 97-99.5% water, hypo-osmotic, slightly acidic solution containing • Electrolytes – Na+, K+, Cl–, PO42–, HCO3– • Digestive enzyme – salivary amylase • Proteins – mucin, lysozyme, defensins, and IgA • Metabolic wastes – urea and uric acid

  17. Teeth • Primary and permanent dentitions have formed by age 21 • Primary – 20 deciduous teeth that erupt at intervals between 6 and 24 months • Permanent – enlarge and develop causing the root of deciduous teeth to be reabsorbed and fall out between the ages of 6 and 12 years • All but the third molars have erupted by the end of adolescence • There are usually 32 permanent teeth

  18. Teeth Figure 24.10.1

  19. Teeth Figure 24.10.2

  20. Classification of Teeth • Teeth are classified according to their shape and function • Incisors – chisel-shaped teeth adapted for cutting or nipping • Canines – conical or fanglike teeth that tear or pierce • Premolars (bicuspids) and molars – have broad crowns with rounded tips and are best suited for grinding or crushing • During chewing, upper and lower molars lock together generating crushing force

  21. Tooth Structure Figure 24.11

  22. Tooth and Gum Disease • Dental caries – gradual demineralization of enamel and dentin by bacterial action • Dental plaque, a film of sugar, bacteria, and mouth debris, adheres to teeth • Acid produced by the bacteria in the plaque dissolves calcium salts • Without these salts, organic matter is digested by proteolytic enzymes • Daily flossing and brushing help prevent caries by removing forming plaque

  23. Tooth and Gum Disease: Periodontitis • Gingivitis – as plaque accumulates, it calcifies and forms calculus, or tartar • Accumulation of calculus: • Disrupts the seal between the gingivae and the teeth • Puts the gums at risk for infection • Periodontitis – serious gum disease resulting from an immune response • Attack of the immune system against intruders: • Also carves pockets around the teeth and • Dissolves bone away

  24. Digestive Processes in the Mouth • Food is ingested • Mechanical digestion begins (chewing) • Propulsion is initiated by swallowing • Salivary amylase begins chemical breakdown of starch • The pharynx and esophagus serve as conduits to pass food from the mouth to the stomach

  25. Deglutition (Swallowing) Figure 24.13a-c

  26. Deglutition (Swallowing) Figure 24.13d, e

  27. Digestion in the Stomach • The stomach: • Holds ingested food • Degrades it both physically and chemically • Delivers chyme to the small intestine • Enzymatically digests proteins with pepsin • Secretes intrinsic factor required for absorption of vitamin B12

  28. Stomach Figure 24.14a

  29. Small Intestine: Gross Anatomy • Runs from pyloric sphincter to the ileocecal valve • Has three subdivisions: duodenum, jejunum, and ileum • The bile duct and main pancreatic duct: • Join the duodenum at the hepatopancreatic ampulla • Are controlled by the sphincter of Oddi • The jejunum extends from the duodenum to the ileum • The ileum joins the large intestine at the ileocecal valve

  30. Microscopic Anatomy of the Small Intestine • Structural modifications of the small intestine wall increase surface area • Plicae circulares: deep circular folds of the mucosa and submucosa • Villi: fingerlike extensions of the mucosa • Microvilli: tiny projections of absorptive mucosal cells’ plasma membranes

  31. Microscopic Anatomy of the Small Intestine Figure 24.21a-c

  32. Liver • The largest gland in the body • Superficially has four lobes – right, left, caudate, and quadrate The lesser omentum anchors the liver to the stomach • The hepatic blood vessels enter the liver at the porta hepatis • The gallbladder rests in a recess on the inferior surface of the right lobe • Bile leaves the liver via • Bile ducts which fuse into the common hepatic duct • The common hepatic duct fuses with the cystic duct • These two ducts form the bile duct

  33. Functions of the liver • Production of bile • Processing bloodborne nutrients • Storage of fat-soluble vitamins • Detoxification of substances

  34. Composition of Bile • A yellow-green, alkaline solution containing bile salts, bile pigments, cholesterol, neutral fats, phospholipids, and electrolytes • Bile salts are cholesterol derivatives that: • Emulsify fat • Facilitate fat and cholesterol absorption • Help solubilize cholesterol • Enterohepatic circulation recycles bile salts • The chief bile pigment is bilirubin, a waste product of heme

  35. Homeostatic Imbalance • Gallstones – crystallization of cholesterol which can obstruct the flow of bile • Current treatments include: dissolving the crystals with drugs, pulverizing them with ultrasound, vaporizing them with lasers, and surgical removal of the gallbladder • Obstructive jaundice – yellowish skin caused by bile pigments deposited in the skin • Due to blocked bile ducts

  36. Liver: Associated Structures Figure 24.20

  37. Pancreas • Location • Lies deep to the greater curvature of the stomach • The head is encircled by the duodenum and the tail abuts the spleen • Exocrine function • Secretes pancreatic juice which breaks down all categories of foodstuff • Acini (clusters of secretory cells) contain zymogen granules with digestive enzymes • The pancreas also has an endocrine function – release of insulin and glucagon

  38. Pancreas Figure 24.26a

  39. Digestion in the Small Intestine • As chyme enters the duodenum • Carbohydrates and proteins are only partially digested • No fat digestion has taken place • Digestion continues in the small intestine • Chyme is released slowly into the duodenum • Because it is hypertonic and has low pH, mixing is required for proper digestion • Required substances needed are supplied by the liver and pancreas • Virtually all nutrient absorption takes place in the small intestine

  40. Large Intestine • Is subdivided into the cecum, appendix, colon, rectum, and anal canal • The saclike cecum: • Lies below the ileocecal valve in the right iliac fossa • Contains a wormlike vermiform appendix

  41. Large Intestine Figure 24.29a

  42. Functions of the Large Intestine • Other than digestion of enteric bacteria, no further digestion takes place • Vitamins, water, and electrolytes are reclaimed • Its major function is propulsion of fecal material toward the anus • Though essential for comfort, the colon is not essential for life