Biliary secretions

BILE SECRETION AND STORAGE.
PHYSIOLGY OF BILIARY TRACT.
PHYSIOLOGY OF BILE
BILIARY SECRETION

1. BILIARY SECRETION BY Prof.Dr . Nusrat Tariq

2. Learning Objectives By end of the lecture you should be able to ; Describe the physiologic Anatomy of Biliary tract. Describe functions of Gall Bladder Describe composition of bile. Enumerate differences between Liver bile and gallbladder bile. Describe emptying of gall bladder. Describe functions of Bile. Describe formation of gallstones

3. BILE Bile secreted continuously by the liver hepatocytes, flows into the duodenum via the CBD.

4. BILE Amount : About 600-1000ml/day . Secretion rate depends on whether a fed or fasting state exist.

5. Storage The Gallbladder is a storage reservoir that can deliver bile to the duodenum for solubilization of dietary lipids

6. Secretion of Bile Bile Is Secreted In Two Stages

7. (1 ) AN INITIAL PORTION: Secreted by the hepatocytes into bile canaliculi . Contains bile acids, cholesterol and other organic constituents. (2) A SECOND PORTION : Is added to the initial bile ( watery solution of sodium and bicarbonate ions ). Secreted by epithelial cells lining the ducts and ductules in its course through the bile ducts. Stimulated by secretin

8. Secretion and enterohepatic circulation of bile salts

9. Storing and Concentrating Bile in the Gallbladder Bile is normally stored in the gallbladder until needed in the duodenum (gallbladder can hold 30 to 60 mL). Gallbladder concentrates the bile by active transport of sodium, and this is followed by secondary absorption of chloride ions, water, and most other diffusible constituents. Bile is concentrated about 5-fold up to a maximum of 20-fold.

10. FORMATION AND FUNCTION OF BILE SALTS The liver cells synthesize about 6 grams of bile salts daily .

11. PRIMARY BILE ACIDS CHOLIC ACID AND CHENODEOXYCHOLIC ACID: The precursor of the bile Acids is cholesterol . The cholesterol is first converted to cholic acid or chenodeoxycholic acid in about equal quantities. These acids in turn combine principally with glycine and to a lesser extent with taurine to form glyco - and tauro -conjugated bile acids . Because they are ionized at neutral pH, conjugated bile acids exist as salts of Na (mainly) or K and therefore are known as Bile Salts .

12. SECONDARY BILE ACIDS Formed by deconjugation nd dehydroxylation of the primary bile salts by intestinal bacteria. F orming D eoxycholic acid from Cholic acid and Lithocholic acid from Chenodeoxycholic acid Lithocholic acid is hepatotoxic and is excreted in urine

13. PRIMARY AND SECONDARY BILE ACIDS

14. FUNCTIONS OF BILE The main digestive function of the liver is the secretion of Bile . Bile serves two important functions : 1- fat digestion and absorption 2- excretion of waste products :

15. 1- FAT DIGESTION AND ABSORPTION Bile salts in the bile perform two functions: Emulsification of the large fat particles. M icelle formation

16. FAT DIGESTION Emulsification They have a detergent action (emulsifying) on the fat particles in the food which decreases the surface tension of the particles. the surface is then attacked by pancreatic lipase

17. FAT ABSORPTION Micelles formation: Help in the absorption of lipids from the intestinal tract by forming small physical complexes with lipids; the complexes are called micelles. The micelles act as a transport medium to carry the monoglycerides and free fatty acids to the brush borders of the intestinal epithelial cells.

18. 2- EXCRETION OF WASTE PRODUCTS : Bile serves as a mean for excretion of waste products from the blood e.g : bilirubin , and excess cholestrol .

19. Secretion and enterohepatic circulation of bile salts

20. Secretion and enterohepatic circulation of bile salts

21. ABSORPTION OF BILE SALTS Occurs largely in ileum where an active transport mechanism exists. In healthy people, the bile acid pool ranges from 2 to 4 g. About 95 % of total pool is reabsorbed.

22. Enterohepatic Circulation of Bile Salts. Means recycling of bile salts between the small intestine and the liver. Bile acid reabsorption occurs mainly in ileum. Bile acids are taken up by hepatocytes from the blood, reconjugated and then resecreted into bile . Bile acids must be recirculated 3-5 time for digestion of a normal meal . Inflammation of the ileum can lead to malabsorption of bile acid and result in the loss of large quantities of bile salts in the feces ( malabsorption of fat).

23. Significance?

24. Enterohepatic Circulation of Bile Salts. By cycling several times during a meal, a relatively small bile acid pool can provide the body with sufficient amounts of bile salts to promote lipid absorption.

25. REGULATION OF BILE SECRETION AND GALL BLADDER EMPTYING

26. BILE SECRETION IS REGULATED BY: 1- Hormonal Control 2- Neural Control 3- Negative feedback from intestine .

27. REGULATION OF BILE SECRETION AND GALL BLADDER EMPTYING 1- Hormonal Control: CCK, Secretin and Estrogen 2 - Neural Control : Parasympathetic ( vagal ) stimulation results in: 1- contraction of the gallbladder 2-relaxation of the sphincter of Oddi , and 3- increased bile formation . Sympathetic stimulation results in: 1- reduced bile secretion 2-relaxation of the gallbladder .

28. 3- Negative feedback from intestine : Bile salts regulate their own synthesis by Negative feedback from intestine Bile salts synthesis is increased with decreased return of bile acids from intestine and is decreased with increased return of bile acids from intestine. If a bile fistula empties the bile salts to the exterior for several days to several weeks so they cannot be reabsorbed from the ileum, the liver increases its production of bile salts 6- to 10-fold,

29. .

30. PH of Bile secreted from Liver ??? PH of Bile from Gall Bladder ???

31. PH of Bile secreted from Liver is Alkaline . PH of Bile from Gall Bladder ??.

32. PH of Bile from Gall Bladder is acidic . WHY? Because most of the HCO3, water and sodium is reabsorbed during storage of bile in gall bladder. Bile acids and the remaining solutes get concentrated in gall bladder .

33. Gall Stone Formation ( cholelithiasis )

34. CAUSES OF GALLSTONES Much more common in women. The demographics are fat, female, fertile, forty. Most stones are cholesterol stones. Cholecystitis . changes the absorptive characteristics of the gallbladder mucosa. Excess water reabsorption . Excess absorption of bile acids from bile. Excess of cholesterol in bile. ratio of bile salts to cholesterol less than 13:1, normal is 20:1

35. CAUSES OF GALLSTONES Cholesterol is insoluble in pure water, but the bile salts and lecithin in bile combine physically with the cholesterol to form ultramicroscopic micelles in the form of a colloidal solution . When the bile becomes concentrated in the gallbladder, the bile salts and lecithin become concentrated along with the cholesterol, which keeps the cholesterol in solution.

36. Bile stasis Use of fibrates e.g. gemfibrozil , which increase cholesterol excretion in bile by up-regulation of beta oxidation of fatty acids, used to decrease serum TAG ( triacylglycerols ) level.

37. TYPES OF GALLSTONES Cholesterol Ca + + billirubinate (hemolytic anemias ) CaCO 3 . Treatment of gallstones Drugs to dissolve the stones e.g. chenodeoxycholic acid to supplement the body’s supply of bile acids, but it takes months to years. Cholecystectomy . Lithotripsy

38. CAUSES OF DECREASED BILE SALTS IN BILE Severe ileal disease Obstruction of biliary tract Severe hepatic dysfunction