Digestion in the Large & Small Intestine

1. Digestion in the Large & Small Intestine

2. Steps of Digestion • Remember there are 4 steps in digestion: • Ingestion • Digestion • Absorption • Egestion • Most digestion & absorption takes place in the small intestine • Lipids • Carbohydrates • Proteins

3. Small Intestine: Structure • Up to 7 m long • 2.5 cm diameter

4. Small Intestine: Structure • Duodenum: most enzymes are added & digestion begins • Jejunum: digestion continues & some nutrients are absorbed • Ileum: most nutrients are absorbed

5. Small Intestine: Structure • Inner surface of small intestine folded into ridges & has many villi • 10 x increase in surface area for nutrient absorption • Microvilli: microscopic projections of cell membrane of epithelial cells that make up villi • Villi + microvilli increase surface area 500 x

6. Small Intestine: Structure • Capillaries: blood vessels that allow nutrients (except fat) to enter the blood stream • Digested fats are transported through lacteals (small vessels) where they enter the lymphatic system and then enter the bloodstream

7. Small Intestine: Structure

8. Chemical Digestion in Small Intestine • Pyloric sphincter controls movement of food (chyme) from stomach to small intestine

9. Pancreas + Digestion • Secretes enzymes needed in digestive process • Secretes hormones that regulate absorption & storage of glucose from blood

10. Pancreas + Digestion

11. Digestion in the Small Intestine • Cholecytoskinin (CCK) secreted in duodenum & released into blood • Signals pancreas to secrete substances that control pH & enzymes needed for lipid, carb., and protein digestion • Signals stomach to slow down digestion so fat can be digested • Secretions enter duodenum through the pancreatic duct

12. Digestion in the Small Intestine • Secretin (hormone) secreted by duodenum that stimulates pancreatic & bile secretions • Pancreas is stimulated to release bicarbonate ions (HCO3-) to neutralize acidic chyme (raise pH from 2.5 – 9.0) • Pepsin inactivated in basic conditions (secretin protects small intestine from stomach acids) • Digestive rate regulator & prevents food from entering duodenum until all is digested

13. Starch Digestion in the Small Intestine • Amylase secreted by pancreas & digests starch in duodenum

14. Protein Digestion in the Small Intestine

15. Lipid Digestion in the Small Intestine • Lipases: enzymes that break down lipids • Break lipid chains into shorter chains & fatty acids • Fat in chyme exists as large globules (lipases cannot penetrate)

16. Liver & Gall Bladder + Digestion • Liver secretes bile (substance that emulsifies fats, breaking them down into tiny micelles) • Lipases can digest micelles of fat • Bile stored in gall bladder until food enters duodenum where it enters duodenum through bile duct • All blood from intestines goes through liver before returning to the heart • Removes toxins • Liver also produces & stores glycogen & fat-soluble proteins

17. Absorption in Small Intestine • The following are absorbed in the small intestine (jejenum & ileum): • Proteins • Carbohydrates • Lipids • Vitamins • Minerals • Water

18. Mechanisms for Absorption • Passive transport • Active transport

19. Passive Transport • Movement of materials across a cell membrane without energy • Diffusion • Osmosis • Facilitated diffusion

20. Diffusion • Particles spread out until evenly distributed • Follows a concentration gradient (high concentration to low concentration) • Amino acids diffuse into blood stream

21. Osmosis • Diffusion of water molecules across a selectively permeable membrane (high concentration of water to low concentration) • Water content in food

22. Facilitated Diffusion • Diffusion of molecules across a membrane through transport proteins (in cell membrane) • Bind to transport molecules on one side of membrane & release them on other side • Only certain substances are allowed to pass through • Carbohydrates

23. Active Transport • Materials moved across cell membrane from low concentration to high concentration using energy from cell • Transport molecules in cell membrane move materials • Transports molecules move materials that are too large to diffuse, or ions that have an uneven charge

24. Passive vs. Active Transport

25. Absorption of Nutrients • All nutrients go from the mucosa of the small intestine into the capillary networks in the villi (bloodstream) • Lacteals carry fats through lymphatic system, to the blood stream • Nutrients are transported from bloodstream into body cells

26. Large Intestine: Structure • 1.5 m long & 7.6 cm in diameter

27. Large Intestine: Structure • Small intestine joins large intestine a few centimetres from the end • Cecum: receives processed materials from small intestine • Appendix: serves no digestive function • Large intestine composed of colon (4 segments: ascending, transverse, descending, sigmoid) & rectum

28. Large Intestine: Function • Digestion is already complete & most nutrients have been absorbed before the large intestine • As undigested matter passes through colon, most of the water is absorbed (osmosis) • Saliva, mucus, gastric juices • Vitamins B & K, Na+ and Cl- are also absorbed • 4 h – 72 h through large intestine depending on type and amount of food consumed

29. Egestion • Removal of waste food materials from body (fibre, cellulose, etc.) • Absorption of water creates feces • Too much water absorbed: constipation • Too little water absorbed: diarrhea • Nerves in large intestine detect movement of feces & urge to have a bowel movement • Feces eliminated when both sphincters are relaxed

30. Summary • http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter26/animation__organs_of_digestion.html • http://kitses.com/animation/swfs/digestion.swf

31. Homework • Page 420 #1, 5, 6, 9