CH 3: Digestion, Absorption, and Transport

1. CH 3: Digestion, Absorption, and Transport The GI Tract

2. Overview Digestive System • Functions: Digestion, Absorption, Elimination • Digestion – process of breaking down foods to release nutrients • Goal is to break nutrients into absorbable units • 2 types of digestions: • Mechanical • Enzymatic (chemical)

3. Overview Digestive System • Absorption – movement of nutrients out of GI tract into blood or lymph • Water soluble nutrients  • Fat soluble nutrients  • Excretion – elimination of undigested foods (feces)

4. Overview Digestive System • Structure Gastrointestinal (GI) Tract • Continuous tube from mouth to anus • See board and page 27

5. GI Tract Anatomy

7. Digestive Tract • Layers of GI tract organs • Serosa (outermost) • Interface between GI tract and lymph & blood • Muscle Layers • Longitudinal muscles • Circular muscles • Submucosal and Mucosal and layers (innermost) • Nerves, blood and lymph vessels • Cells of the mucosal layer produce secretions

8. Mouth • Structure – teeth, tongue, salivary glands • Secretions • Saliva • Salivary amylase ….. • Digestion • Mechanical …. • Enzymatic/chemical …. • Swallow  tongue pushes food  pharynx  ……

11. Esophagus • Structure – 12” tube • UES and LES • Function • Transports food from mouth to stomach • Peristalsis and gravity aid food movement • Secretions -- mucus • Digestion • Mechanical (limited) • Enzymatic/chemical – starch digestion continues

13. Stomach • Structure ….page 73 • Secretions ….. • Digestion …… • Mechanical • Enzymatic/chemical

14. Stomach • Structure – muscular sack that can expand • extra muscle layer to aid in the mechanical digestion of food (pg 73)

15. Stomach Related Secretions • Gastrin – hormone that stimulates stomach to release secretions • Hydrochloric Acid (HCl) -- unravels proteins, kills bacteria, activates pepsinogen • Pepsinogen – once activated, begins protein digestion • Mucus – secreted by goblet cells, protects stomach, moistens food

17. Stomach - Digestion • Mechanical • Stomach muscles grind food into a paste called chyme • Enzymatic/chemical • Proteins uncoiled • Protein digestion to polypeptides begins • Starch digestion stops (why??)

18. Small Intestine (SI) • Function – The SI is where: • the majority of digestion to absorbable units occurs • Digestion of carbohydrates, fats, and proteins • Vitamins and minerals do not need digestion • Nutrients are absorbed into either capillaries or lacteals

19. Small Intestine • Structure – see page 79 • Length -- ~ 21’-22’ long when relaxed • Layers ….. • Mucosal folds, villi, microvilli …. • Goblet cells and crypts – create secretions • Lacteals and capillaries …. • Sections • Duodenum • Jejunum • Ileum

20. Small Intestine

21. The Small Intestinal Villi

22. The Small Intestinal Villi

23. Small Intestines • Secretions of the SI • Mucus • Secreted by ____________ cells • Digestive enzymes that finish the digestion of carbs, fats, and proteins • Secreted from crypts

24. Small Intestine • Secretions of the SI, cont’d • Hormones • Secretin … • CCK …. • Gastric-inhibitory peptide …

25. Small Intestine Hormone Summary • Secretin – produced when chyme enters SI • stimulates pancreatic secretions • *CCK – produced when fat enters SI • Stimulates _________to release ______ • Slows GI motility (slows peristalsis) • *Cholecystokinin • Gastric–inhibitory peptide– produced when chyme enters SI • Slows stomach secretions • Slows GI motility

27. Secretions into SI • Pancreatic secretions: • Released in response to ________ • Sodium bicarbonate • Neutralizes acidic chyme • Digestive enzymes that begin the digestion of carbs, fats, and proteins

28. Secretions into SI • Liver and Gall Bladder • Liver makes bile • Gall bladder concentrates and stores bile • Bile is released into SI in response to the hormone _______ • Function of bile:

30. Small Intestine • Digestion - Mechanical • Peristalsis pushes food through SI • Segmentation mixes chyme with digestive enzymes • Also breaks up food mass into smaller masses • Bile emulsifies fats

31. Peristalsis & Segmentation

32. Muscular Action of Digestion Segmentation

33. SI • Digestion – Chemical/Enzymatic • Pancreatic and SI enzymes digest carbs, fats, and proteins to absorbable units • Duodenum • Jejunum • Ileum

34. SI • Absorbed into capillaries (blood) • Digested carbohydrates and proteins • Minerals • Some require helper proteins/cells on walls of SI to be absorbed • Water soluble vitamins • Blood takes nutrients to the liver for processing after absorption (pg 81)

35. SI • Absorbed into lacteals (lymph) • Digested fats • Fat soluble vitamins • Cholesterol • Nutrients travel through lymph system to chest area where lymph and blood join • Nutrients enter blood and travel to liver

37. SI Review What is the relationship between the structure of the SI and its function?

38. Large Intestine or Colon • Undigested foods (fiber) enter into colon • Unabsorbed nutrients pass into colon • E.g. calcium, iron, cholesterol trapped in fiber

39. Colon • Structure • 5-6’ long • Wider diameter than SI • No villi or microvilli • Appendix is a little pouch near beginning of colon • Colon “wraps around” SI • Page 82

41. Colon • Secretions • Mucus • Bacteria living in colon • Digest small amounts of fiber and undigested nutrients • Often produce…...

42. Colon • Absorption • Water, salts, vitamins made by bacteria are absorbed into __________ • Fiber attracts water • Too little fiber in diet  • Too much fiber in diet 

43. Rectum • Feces pass from colon into rectum • Rectum stores feces until excretion occurs