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  1. 1. Digestive System
  2. 2. The Digestive System <ul><li>Gastrointestinal (GI) tract or alimentary canal </li></ul><ul><ul><li>Continuous tube from mouth to anus </li></ul></ul><ul><ul><li>Mouth, Pharynx, Esophagus, Stomach, Small intestine, and Large intestine </li></ul></ul><ul><ul><li>Length in cadaver 30ft., shorter in living due to muscle contraction </li></ul></ul><ul><li>Accessory digestive organs </li></ul><ul><ul><li>teeth, tongue, salivary glands, liver, gallbladder, and pancreas </li></ul></ul>
  3. 4. Overview of GI tract Functions <ul><li>Mouth - bite, chew, swallow </li></ul><ul><li>Pharynx and esophagus - transport </li></ul><ul><li>Stomach - mechanical digestion, absorption of water & alcohol </li></ul><ul><li>Small intestine - chemical & mechanical digestion & absorption </li></ul><ul><li>Large intestine - absorb electrolytes & vitamins (B and K) </li></ul><ul><li>Rectum and anus---defecation </li></ul>
  4. 5. Processes of Digestive System <ul><li>Ingestion is taking food into the mouth (eating). </li></ul><ul><li>Defecation is emptying of the rectum, eliminating indigestible substances from the GI tract. </li></ul>
  5. 6. Digestion <ul><li>Digestion breaks food into small molecules to facilitate absorption </li></ul><ul><ul><li>Polysaccharide – Monosaccharide </li></ul></ul><ul><ul><li>Proteins - Amino acids </li></ul></ul><ul><ul><li>Fats - fatty acids and glycerol </li></ul></ul><ul><ul><li>By hydrolysis </li></ul></ul><ul><ul><li>Aided by enzymes – increases rate at body temp </li></ul></ul>
  6. 7. Chemical Digestion <ul><li>Secretion is the release, by cells within the walls of the GI tract and accessory organs, of water, acid, buffers, and enzymes into the lumen of the tract (total of 7 liters). </li></ul>
  7. 8. Mechanical Digestion <ul><li>Mixing and propulsion result from the alternating contraction (peristalsis) and relaxation of the smooth muscles within the walls of the GI tract. </li></ul><ul><li>Aids chemical digestion </li></ul>
  8. 9. <ul><li>Absorption is the passage of end products of digestion from the GI tract into blood or lymph for transport to cells. </li></ul><ul><li>Assimilation occurs when food molecules become part of body tissues/metabolic processes </li></ul>
  10. 11. Histology of Digestive Tract <ul><li>Epithelial layer is stratified squamous in mouth, esophagus, and anal canal – protection </li></ul><ul><li>Stomach and intestines are lined by simple columnar epithelium – secretion and absorption </li></ul>
  11. 12. Digestion in the Mouth <ul><li>Mechanical digestion (mastication or chewing) </li></ul><ul><li>Chemical digestion </li></ul><ul><ul><li>Salivary amylase </li></ul></ul><ul><ul><ul><li>begins starch digestion at pH of 7.0 </li></ul></ul></ul><ul><ul><ul><li>swallowed too quickly for all starches to break down; Continues to digest for another hour; Stomach acid inactivates </li></ul></ul></ul><ul><ul><ul><li>Substrate – starch </li></ul></ul></ul><ul><ul><ul><li>Product – maltose and glucose </li></ul></ul></ul>
  12. 14. Composition and Functions of Saliva <ul><li>99.5% water and 0.5% solutes </li></ul><ul><ul><li>Bicarbonate ions buffer acidic foods (pH 6.35-6.85) </li></ul></ul><ul><ul><li>Chemical digestion of starch begins with enzyme (salivary amylase) </li></ul></ul><ul><ul><li>Mucus lubricates food </li></ul></ul><ul><ul><li>Enzyme (lysozyme) ---helps destroy bacteria </li></ul></ul>
  13. 15. Structure and Function of the Tongue <ul><li>Muscles move tongue to assist in chewing and swallowing </li></ul><ul><li>Papillae are the bumps---taste buds </li></ul><ul><li>Lingual glands secrete mucus and serous fluid with enzyme lipase </li></ul>
  14. 16. Dentition <ul><li>Primary or baby teeth </li></ul><ul><ul><li>20 teeth that start erupting at 6 months </li></ul></ul><ul><ul><li>1 new pair of teeth per month </li></ul></ul><ul><li>Permanent teeth </li></ul><ul><ul><li>32 teeth that erupt between 6 and 12 years of age </li></ul></ul><ul><ul><li>incisors for biting, canines for tearing, premolars & molars for crushing and grinding food </li></ul></ul><ul><ul><li>Often not enough room for third molars (wisdom teeth) </li></ul></ul>
  15. 17. Esophagus <ul><li>Collapsible muscular tube, passageway </li></ul><ul><li>10 inches long </li></ul><ul><li>Anterior to vertebrae </li></ul><ul><li>Posterior to trachea </li></ul><ul><li>Posterior to the heart </li></ul>
  16. 18. Disorders <ul><li>Achalasia – lower esophageal sphincter fails to relax, meal stuck in esophagus </li></ul><ul><li>Heartburn – lower esophageal sphincter fails to close after food enters stomach </li></ul>
  17. 19. Stomach--M echanical Digestion <ul><li>peristaltic movements called mixing waves. </li></ul><ul><li>Pyloric sphincter remains almost, but not completely, closed </li></ul><ul><li>Gastric emptying – about 3mL of chyme is forced out </li></ul><ul><li>Most chyme is forced back into body of stomach to continue mixing </li></ul>
  18. 20. Stomach--Chemical Digestion <ul><li>Secretion of HCl </li></ul><ul><ul><li>inactivates amylase </li></ul></ul><ul><ul><li>activates pepsinogen by converting it to pepsin </li></ul></ul><ul><ul><ul><li>Optimum pH of 1-2 </li></ul></ul></ul><ul><ul><ul><li>Substrate – proteins </li></ul></ul></ul><ul><ul><ul><li>Product – smaller polypeptides and amino acids </li></ul></ul></ul><ul><ul><li>kills microbes </li></ul></ul><ul><ul><li>stimulates secretion of hormones that promote bile and pancreatic juice </li></ul></ul>
  19. 21. Absorption in the Stomach <ul><li>Epithelial cells are impermeable to most materials </li></ul><ul><li>Water </li></ul><ul><li>Electrolytes </li></ul><ul><li>Short-chain fatty acids </li></ul><ul><li>Some drugs (especially aspirin) & alcohol </li></ul><ul><li>Gastric mucosal cells contain alcohol dehydrogenase that converts some alcohol to acetaldehyde-----more of this enzyme found in males than females </li></ul>
  20. 22. Anatomy of Stomach
  21. 23. Stomach Lining
  22. 25. Disorders <ul><li>Peptic ulcer Disease </li></ul><ul><ul><li>H. pylori bacterium </li></ul></ul><ul><ul><li>Pain relievers </li></ul></ul><ul><ul><li>Hypersecretion of HCl </li></ul></ul>
  23. 26. Composition and Functions of Pancreatic Juice <ul><li>1 & 1/2 Quarts/day at pH of 7.1 to 8.2 </li></ul><ul><li>Contains water, enzymes & sodium bicarbonate </li></ul><ul><li>Digestive enzymes </li></ul><ul><ul><li>pancreatic amylase </li></ul></ul><ul><ul><li>pancreatic lipase </li></ul></ul><ul><ul><ul><li>Optimum pH 7 </li></ul></ul></ul><ul><ul><ul><li>Substrate – lipids </li></ul></ul></ul><ul><ul><ul><li>Product – fatty acids and glycerol </li></ul></ul></ul><ul><ul><li>Pancreatic nucleases </li></ul></ul><ul><ul><li>Pancreatic trypsin (protease) </li></ul></ul>
  24. 27. Histology of the Pancreas <ul><li>Acini </li></ul><ul><ul><li>Exocrine </li></ul></ul><ul><ul><li>99% of gland </li></ul></ul><ul><li>Islets of Langerhans </li></ul><ul><ul><li>Endocrine </li></ul></ul><ul><ul><li>1% of gland </li></ul></ul>
  25. 28. Bile Production <ul><li>One quart of bile/day is produced by the liver </li></ul><ul><ul><li>yellow-green in color & pH 7.6 to 8.6 </li></ul></ul><ul><li>Components </li></ul><ul><ul><li>Water, cholesterol, bile salts, bile pigments (bilirubin) from hemoglobin molecule </li></ul></ul><ul><li>Functions </li></ul><ul><ul><li>Emulsification </li></ul></ul><ul><li>Produced by hepatocytes in liver, stored in gallbladder </li></ul>
  26. 29. Anatomy of the Small Intestine <ul><li>Major events of digestion and absorption occur in small intestine </li></ul><ul><li>Large surface area </li></ul><ul><li>3 parts </li></ul><ul><ul><li>duodenum---10 inches </li></ul></ul><ul><ul><li>jejunum---8 feet </li></ul></ul><ul><ul><li>ileum---12 feet </li></ul></ul><ul><li>Small diameter </li></ul>
  27. 30. Histology of Small Intestine
  28. 31. Cells of Intestinal Glands
  29. 32. Chemical Digestion in Small Intestine <ul><li>Chyme entering small intestine contains partially digested carbohydrates, proteins, and lipids </li></ul><ul><li>The completion of the digestion is a collective effort of pancreatic juice, bile, and intestinal juice in the small intestine </li></ul><ul><li>Occurs in duodenum </li></ul>
  30. 33. Review: Digestion of Carbohydrates <ul><li>Mouth---salivary amylase </li></ul><ul><li>Duodenum----pancreatic amylase </li></ul><ul><li>Brush border enzymes (maltase, sucrase & lactase) act on disaccharides </li></ul><ul><ul><li>produces monosaccharides--fructose, glucose & galactose </li></ul></ul><ul><ul><li>lactose intolerance (no enzyme; bacteria ferment sugar)--gas & diarrhea </li></ul></ul>
  31. 34. Review: Digestion of Proteins <ul><li>Stomach </li></ul><ul><ul><li>HCl denatures or unfolds proteins </li></ul></ul><ul><ul><li>pepsin turns proteins into small polypeptides </li></ul></ul><ul><li>Pancreas </li></ul><ul><ul><li>Pancreatic trypsin--split peptide bonds between amino acids </li></ul></ul><ul><li>brush border enzymes-----aminopeptidase or dipeptidase </li></ul>
  32. 35. Review: Digestion of Lipids <ul><li>Mouth----lingual lipase </li></ul><ul><li>Most lipid digestion, in an adult, occurs in the small intestine. </li></ul><ul><ul><li>emulsification by bile </li></ul></ul><ul><ul><li>pancreatic lipase---splits triglycerides into fatty acids & monoglycerides </li></ul></ul>
  33. 36. Digestion of Nucleic Acids <ul><li>Nucleic acids are broken down into nucleotides for absorption. </li></ul><ul><li>Pancreatic juice contains 2 nucleases </li></ul><ul><li>Nucleotides produced are further digested by brush border enzymes </li></ul>
  34. 37. Where will the absorbed nutrients go?
  35. 38. Anatomy of Large Intestine 5 feet long by 2½ inches in diameter
  36. 39. Disorders <ul><li>Appendicitis – inflammation of appendix </li></ul><ul><ul><li>Due to blockage or bacterial infection </li></ul></ul><ul><ul><li>Can rupture and spread to abdomen </li></ul></ul>
  37. 40. Histology of Large Intestine <ul><li>Mucosa </li></ul><ul><ul><li>smooth tube -----no villi or plica </li></ul></ul><ul><ul><li>Micro-villi of absorptive cells are present </li></ul></ul><ul><ul><li>simple columnar cells absorb water & goblet cells secrete mucus </li></ul></ul>
  38. 41. Digestion in Large Intestine <ul><li>Mechanical digestion includes peristalsis </li></ul><ul><li>Chemical digestion - bacteria </li></ul><ul><ul><li>undigested carbohydrates into carbon dioxide & methane gas </li></ul></ul><ul><ul><li>undigested proteins into simpler substances (odor) </li></ul></ul><ul><ul><li>turn bilirubin into simpler substances that produce color </li></ul></ul><ul><ul><li>Bacteria produce vitamin K and B in colon </li></ul></ul>
  39. 42. Absorption & Feces Formation <ul><li>Some electrolytes (Na+ and Cl-) and vitamins </li></ul><ul><li>After 3 to 10 hours, Feces are semisolid </li></ul><ul><li>Feces = dead epithelial cells, undigested food such as cellulose, bacteria (live & dead) </li></ul><ul><li>90% of H2O has been removed from chyme in small intestine, but large intestine absorbs enough water to make it important in maintaining water balance </li></ul>
  40. 43. Absorption of W ater <ul><li>9 liters of fluid dumped into GI tract each day </li></ul><ul><li>Small intestine reabsorbs 8 liters </li></ul><ul><li>Large intestine reabsorbs 90% of that last liter </li></ul>
  41. 44. Timing <ul><li>After food is chewed and swallowed – 5-10 seconds to pass down esophagus to stomach </li></ul><ul><li>Partial digestion in stomach – 2-6 hours </li></ul><ul><li>Final digestion and absorption in small intestine – 5-6 hours </li></ul><ul><li>In 12-24 hours undigested material passes through large intestine </li></ul>