Chapter 25 - 3

1. Chapter 25 - 3 The Digestive System

2. The Gall bladder • Hollow, pear-shaped organ • Stores, modifies and concentrates bile • Contraction of the gall bladder and release of bile is controlled by CCKand vagal stimulation • Empties into the Cystic duct • Gall stones

3. The Gallbladder

4. Bile • Made of water, billirubin, ions, cholesterol, other lipids • Acts as an emulsifier to aid lipid digestion • 90% is reabsorbed in the ileum - enterohepatic circulation of bile

5. Functions of the large intestine • Reabsorb water and compact material into feces • 1500 mL chyme enter the cecum, 90% of volume reabsorbed yielding 80-150 mL of feces • Absorb vitamins produced by bacteria • Store fecal matter prior to defecation

6. General anatomy of the large intestine • Lies inferior to the stomach • Frames the small intestine • Parts of the large intestine • Cecum • Colon • Ascending • Transverse • Descending • Sigmoid • Rectum • Anal canal

7. The Large Intestine

8. Cecum and Rectum • Cecum • opening protected by ileocecal valve • veriform appendix • Rectum • Last portion of the digestive tract • Terminates at the anal canal • Internal anal sphincter - involuntary smooth muscle • External anal sphincter - voluntary muscle • Hemorrhoids -inflamed veins

9. Rectum

10. Histology of the large intestine • Columnar cells except the rectum and anal canal (stratified squamous) • Muscularis externa • circular muscle • only 3 bands longitudinal smooth muscle - taeniae coli • Absence of villi • Presence of goblet cells • Deep intestinal glands

11. Physiology of the large intestine • Absorption in the large intestine includes: • Water • Vitamins – K, biotin, and B5 • Organic wastes – urobilinogens and sterobilinogens • Bile salts • Some ammonia and other toxins • Mass movements of material through colon and rectum • Defecation reflex triggered by distention of rectal walls

12. Movement in Large Intestine • Mass movements • Common after meals • Integrated by the enteric plexus • Local reflexes • instigated by the presence of food in the stomach and duodenum • Gastrocolic reflex: initiated by stomach • Duodenocolic reflex: initiated by duodenum

13. Long reflexes of defecation

14. Coordination secretion and absorption Neural and hormonal mechanisms coordinate glands GI activity stimulated by parasympathetic innervation Inhibited by sympathetic innervation Enterogastric, gastroenteric and gastroileal reflexes coordinate stomach and intestines Several hormones alter motility and secretion

15. Summary Hormone Table GIP duodenum fats, carbohydrates stim pancreas insulin secretion, inhibits stomach, stim adipose glucose and FA uptake & lipogenesis Marieb Table 23.1.1

16. Summary Hormone Table (continued) Marieb Table 23.1.1

17. Activities of Major Digestive Tract Hormones From Martini Figure 24.22

18. Digestion, Absorption, Transport • Digestion • Disassembles organic food into smaller fragments • Mechanical: breaks large food particles to small • Chemical: breaking of covalent bonds by digestive enzymes • Hydrolyzes carbohydrates, proteins, lipids and nucleic acids for absorption • Absorption and transport • Molecules are moved out of digestive tract and into circulation for distribution throughout body

19. Carbohydrate digestion and absorption • Begins in the mouth (salivary amylase) completed in small intestine • Salivary and pancreatic amylase • Convert starches to disaccharides, trisaccharides, monosaccharides • Brush border enzymes • Make monosaccharides from disaccharides • maltase (glucose / glucose) • sucrase (glucose / fructose) • lactase (glucose / galactose) • Absorption of monosaccharides occurs across the intestinal epithelia by facilitaed diffusion or Na+ linked co-transporters

20. Lipid digestion and absorption • Begins in stomach (minor) completed in small intestine • Lipid digestion utilizes lingualand pancreatic lipasesconverts triglycerides to monoglycerides • Bile salts improve chemical digestion by emulsifying lipid drops • Lipid-bile salt complexes called micelles are formed • Micelles diffuse into intestinal epithelia which re-synthesis triglycerides and then release lipids into the circulation as chylomicrons - mixtures of phopholipids and proteins • carried by lymphatic system

21. Lipid Absorption also (see Saladin fig 25.30) from Seeley, Stephens and Tate

22. Protein digestion and absorption (See Saladin Fig 25.29) • In the stomach, the low pH destroys tertiary and quaternary structure allows pepsin to digest specific peptide bonds • In the small intestine, pancreatic enzymes and intestinal brush border peptidases work at pH 7-8 • Trypsin - breaks bonds next to arginine or lysine • Chymotrypsin- breaks bonds next to phenylalanine or tyrosine • Liberated amino acids, and some dipeptides are absorbed through by several different carrier proteins via by facilitaed diffusion or Na+ linked co-transporters

23. Absorption of other molecules • Water • Nearly all that is ingested is reabsorbed via osmosis • Ions • Absorbed via diffusion, cotransport, and active transport • Vitamins • Water soluble vitamins (C and B vitamins) are absorbed by diffusion • Vitamin B12 requires intrinsic factor • Fat soluble vitamins (A,D, E, K) are absorbed as part of micelles

24. Water Intake, Secretion and Absorption Diarrhea Constipation from Seeley, Stephens and Tate

25. Great Summary slide (a) Marieb Figure 23.33a

26. Great Summary slide (b) Marieb Figure 23.33b