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Physiology of the Gastrointestinal Tract
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  1. Physiology of the Gastrointestinal Tract Dr. Sherwan Rahman Sulaiman www.doctorsherwan.com

  2. What are the Functions of the Gut? • Digestion… Process by which large molecules in diet are broken down into smaller ones, which are acceptable to the enterocytes for absorption.

  3. What are the Functions of the Gut? • Absorption…Process by which contents of the small bowel enter the mucosal epithelial cells, and eventually the portal vein or lymphatics.

  4. What are the Functions of the Gut? • Protection… Barrier to entry of pathogens and toxins

  5. Anatomy and Physiology of Digestive System - Mouth • Mouth - Digestive Action • Food mechanically broken down • Saliva - normally about 25 ml/hr is secreted, increases up to 300 ml per hour with eating • Salivary a-amylase degrades starch, a-amylase is deactivated by gastric acid

  6. Anatomy and Physiology of Digestive System - Mouth • Mouth - Protective Action • Preventive bacteria in the mouth are important for defense against invading microorganisms • Contains specific antimicrobial proteins like lysozyme, lactoferrin and lactoperoxidase, but also mucin, IgA, and nitric oxide-donating substances such as nitrates • Mucus covers food and follows it to colon. It can attach to mucosal surfaces and forms protective barrier

  7. Anatomy and Physiology of Digestive System - Mouth • Inhibition of Saliva • Anticholinergics, analgesics, antispasmodics, antidiarrheals, antidepressants, antihistamines, antihypertensives, antipsychotics, and diuretics • Stimulation of Saliva • Pilocarpine 5mg po tid

  8. Anatomy and Physiology of Digestive System - Esophagus • Esophagus • Transports food to stomach • Pharyngeal lipase hydrolyzes triglycerides to diglycerides and fatty acids

  9. Anatomy and Physiology of Digestive System - Stomach Stomach: Stores, mixes, and grinds food to form an emulsion Fundus Gastroesophageal Junction Cardia Incisura Body Pylorus Antrum Duodenal Bulb

  10. Anatomy and Physiology of Digestive System - Stomach Hydrochloric acid from parietal cells denatures protein Pepsinogen (Pepsin) from zymogen (chief) cells begins proteolysis Surface Epithelial Cell Mucous Cell Parietal Cell Zymogen Cell Lymph Nodule Argentaffine Cell Muscularis Mucosae Submucosa Pyloric Glands Gastric or Fundic Glands

  11. Anatomy and Physiology of Digestive System - Stomach Pepsin Acid Gastric lumen pH 1-3 Mucous gel layer Bicarbonate / Mucus pH 7 Gastric mucosa Zymogen Cell Mucous Cell Parietal Cell

  12. Anatomy and Physiology of Digestive System - Small Bowel • Small Intestine - averages around 5 meters in length • Duodenum: ~10” long, 2” in diameter • Jejunum: ~ 2/5 length of rest of small bowel, 1 ½ to 1 ¼” in diameter, thick, many blood vessels. • Ileum: ~3/5 length, 1 ¼ to 1” diameter, thin wall, large Peyer’s patches

  13. Anatomy and Physiology of Digestive System - Small Bowel • Small Intestine • Major organ for nutrient absorption • Absorptive surface enhanced by plicae circulares, foldings called villi with surface projections called microvilli • Final surface area about 1.7 m cm2 • About 800cm2 to absorb 1 Kcal (100-200 cm2/Kcal minimum)

  14. Anatomy and Physiology of Digestive System - Small Bowel Plica circularis or Valve of Kerckring Villus Jejunum Ileum

  15. Anatomy and Physiology of Digestive System - Colon • Absorbs water and electrolytes • Stores waste • “Organ within an Organ” Colonic microflora ferment malabsorbed nutrients and soluble fiber to a form the colonic mucosa can absorb

  16. Absorption of Water • 8 L/day fluid reaches the small intestine of which about 2 L is dietary in origin • Small bowel absorbs about 7 L/d, mainly in jejunum, colon absorbs 1 to 1.5 L/d (can increase absorption up to 4 L/d)

  17. Absorption of Water • Water absorption is entirely passive - follows absorption of solutes (can move in either direction, depending on osmotic gradients) • Particularly true of the solute Na+ • High sodium enteral diets enhance water absorption • Low sodium diets predispose to diarrhea

  18. Absorption of Sodium and Chloride - Small Bowel • Sodium absorption is directly coupled to absorption of organic solutes such as glucose, amino acids, water-soluble vitamins, and bile salts

  19. Absorption of Sodium and Chloride - Small Bowel • Once inside cell, sodium is extruded against chemical and electrical gradient via a basolateral membrane-associated Na+-K+-ATPase

  20. Absorption of Sodium and Chloride - Small Bowel • Chloride passively follows absorption of sodium

  21. Absorption of Sodium and Chloride – Distal Ileum and Colon • Neutral NaCl co-transport, Na+ for H+ and Cl for HC03

  22. Absorption of Potassiumin Small Bowel • Overall K+ movement is result of solvent drag and is potential-dependent • K+ actively secreted in colon • Rectosigmoid colon has active K+ absorption - exchanges K+ for H+

  23. Absorption of Calcium • Passive - throughout the small intestine • Predominates at concentrations ³ 10 mmol/L • Active - primarily in the duodenum • Below 10 mmol/L, active transport occurs with 1,25 dihydroxy vitamin D3

  24. Absorption of Calcium

  25. Fiber Digestion and Absorption • Non-starch carbohydrate of plant origin that escapes enzymatic digestion in the small intestine • Two types • Cellulosic: high molecular weight, non-soluble (cellulose, wheat bran) • Noncellulosic: soluble (hemicelluloses, pectin, gums, mucilages)

  26. Triglycerides • Majority of triglycerides contain long-chain fatty acids (16 -18 C) • A few dietary triglycerides contain medium-chain fatty acids (8 -12 C)

  27. Lipid Absorption • Lipids in diet are emulsified in the stomach by mechanical grinding • Pancreatic lipase in duodenum digests to free fatty acids and 2-monoglycerides • Bile acids form water soluble micelles which diffuse easily across the unstirred water layer

  28. Fat Digestion and Absorption • Medium-chain triglycerides, which are more water soluble, may be absorbed intact with direct transport to the portal system as free fatty acids

  29. Protein Digestion and Absorption • Derived from animal and vegetable sources and make up to 11 to 14% of average caloric intake (70 to 100 gm/day) • Primarily absorbed in the duodenum and proximal jejunum, yet some does pass into and is absorbed by the colon