Anatomy and Physiology of the Gastrointestinal Tract

1. Anatomy and Physiology of the Gastrointestinal Tract John P. Grant, MD, CNSP Director Nutrition Support Service Professor of Surgery Duke University Medical Center Durham, NC

2. Specialty Examination • Rule of thirds: • 1/3 of questions are simple – no study needed. • 1/3 of questions you will answer from experience. • 1/3 of questions you will not know, studying will not help (with combination of studying and experience you can make an educated guess in 1/3).

3. 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.

4. 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.

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

6. Digestion and Absorption - ??? • What digestive process does not occur in the oropharynx? • Micelles are formed with fat • Salivary a-amylase degrades starch • Pharyngeal lipase hydrolyzes triglycerides to diglycerides and fatty acids • Food particles are mechanically broken up

7. Digestion and Absorption - ??? • What digestive process does not occur in the oropharynx? • Micelles are formed with fat • Salivary a-amylase degrades starch • Pharyngeal lipase hydrolyzes triglycerides to diglycerides and fatty acids • Food particles are mechanically broken up

8. 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

9. 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

10. 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

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

12. 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

13. 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

14. 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

15. 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

16. 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)

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

18. Digestion and Absorption - ??? • The absorptive surface of the normal small intestine is equal to: Poor question… • 3 tennis courts • 5 tennis courts • 1 tennis court • 10 tennis courts

19. Digestion and Absorption - ??? • The absorptive surface of the normal small intestine is equal to: Poor question… • 3 tennis courts • 5 tennis courts • 1 tennis court • 10 tennis courts

20. Digestion and Absorption - ??? • How much small intestine can be removed before a patient will develop short bowel syndrome? Poor question…

21. Digestion and Absorption - ??? • How much small intestine can be removed before a patient will develop short bowel syndrome? Poor question…

22. Digestion and Absorption - ??? A patient is likely to, but not always, require HTPN if only the following amount of small bowel can be saved at the time of surgery: It is always better to know the amount of bowel remaining following surgical resection – rather than the amount resected…Better question:

23. Digestion and Absorption - ??? A patient is likely to, but not always, require HTPN if only the following amount of small bowel can be saved at the time of surgery: It is always better to know the amount of bowel remaining following surgical resection – rather than the amount resected…Better question:

24. Digestion and Absorption There is a high probability of transitioning patients off HTPN if the ratio of remaining intestinal length to body weight is: > 0.5 cm/kg ie: > 40 cm for an 80 kg patient Wilmore, et al., Ann. Surg., 226:288-293, 1997

25. 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

26. Nutrient Absorption

27. Sites of Nutrient Absorption

28. Sites of Nutrient Absorption Good question… • What nutrient deficiency might be expected if 3 feet of the terminal ileum is resected?

29. Sites of Nutrient Absorption Good question… • What nutrient deficiency might be expected if 3 feet of the terminal ileum is resected?

30. Sites of Nutrient Absorption Much harder question… • What nutrient deficiency might be expected if 3 feet of the terminal ileum is resected?

31. 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)

32. Digestion and Absorption - ??? • Water absorption by the small bowel may be enhanced by adding the following to the enteral formula:

33. Digestion and Absorption - ??? • Water absorption by the small bowel may be enhanced by adding the following to the enteral formula:

34. 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

35. 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

36. 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

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

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

39. 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+

40. 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

41. Absorption of Calcium

42. Absorption of Magnesium • Absorbed from distal small intestine and all of colon by passive diffusion

43. Digestion and Absorption - ??? • What mineral will be depleted if a patient has a draining T-tube in the common bile duct ? Good question…

44. Digestion and Absorption - ??? • What mineral will be depleted if a patient has a draining T-tube in the common bile duct ? Good question…

45. Absorption of Copper • Dietary copper is absorbed in stomach and duodenum • Active process requires energy and involves absorption of complexes of copper and amino acids • Copper is excreted by bile urine losses = ~1-2% of intake

46. Digestion and Absorption - ??? • What mineral will be depleted if a patient has marked diarrhea ? Good question…

47. Digestion and Absorption - ??? • What mineral will be depleted if a patient has marked diarrhea ? Good question…

48. Absorption of Zinc • Major loss of zinc is in the feces - during intravenous nutrition it is suggested to give: 2 mg Zn + 17.1 mg Zn/kg stool lost + 12.2 mg Zn/kg of gastric/duodenal/or jejunal fluid lost

49. Absorption of Zinc • Zinc is absorbed primarily in jejunum • Binds to a ligand in lumen – transports to mucosa • Transferred to binding site on cell • Active process requiring energy, oxygen, and Na+ • Absorption stimulated by glucose

50. Digestion and Absorption - ??? • Even in Short Bowel Syndrome, oral iron supplementation can be effective. Good question… • True • False