55. Production, functions and regulation of the bile

Question 1

Functions

Answer 1

• Bile salts take part in emulsifying fat (secretory)

* Metabolic end-products and antibodies (excretory)

Question 2

Components

Answer 2

• Bile consists of: bile salts, cholesterol and lecithin which form micelles.
• Conjugated bile acids form salts with different cations (mainly sodium), which are called bile salts
• Lecithin has an amphiphatic character (hydrophobic + hydrophilic) and is found at the borders
• Cholesterol is hydrophobic and is found inside the micelle
• Bile also contains pigments and electrolytes (bicarbonate)
• Lecithin increase the solubility of cholesterol, but if cholesterol level increases, the extra amount may form a crystal.

Question 3

Production and circulation of bile acids

Answer 3

• Bile acids are synthesized from cholesterol, and forms up to 50% of the dry mass of bile
• Primary bile acids are cholic acids and chenodeoxycholic acid.

Question 4

Conjugation of primary bile acids

Answer 4

• Primary bile acids can be conjugated with taurine and glycine in the liver cells: glyco- or taurocholic acid is formed.
• The carboxyl group of cholic acid and chenodeoxycholic acid dissociate in alkaline pH, the conjugated bile acids dissociate at physiological pH.
• This increases the water solubility and efficiency, and bile acids have emulsified lipids, and lipids are absorbed by the time they reach the ileum.
• Primary bile acids are dehydroxylated and deconjugated by bacteria in the terminal end of ileum, and secondary bile acids are generated: deoxycholic acid (from cholic acid) and lithocholic acid (from chenodeoxycholic acid)
• This increases their lipid solubility, and therefore increases their absoption with passive diffusion → enters the enterohepatic circulation

Question 5

Enteroheptatic circulation

Answer 5

• 95-98% of bile salts are absorbed actively in the ileum, and gets into the liver via the portal vein, where hepatocytes take them up, conjugate them again, or hydrolyses the secondary bile acids and secrete them into the bile.
• A smaller portion (5%) is absorbed passively from the jejunum.
• Bile salt transported to the liver by the enterohepatic circulation inhibit the bile acid synthesis from cholesterol → repression mechanism

Question 6

Regulation of bile secretion

Answer 6

• N. vagus: Gall bladder slightly contract due to parasympathetic effect of vagus
• Secretin: Stimulates secretion of bile to the intestine, but not the bile production by the parenchyma cells itself. Result in bile salt dilution, containing a rather high concentration of bicarbonate. Secretin has a main role in buffering the acetic intestinal content.
• Cholecystokinin: High lipid content of the chime stimulates CCK production of the intestinal cells. Cause contraction of gall bladder, as well as relaxation of sphincter = increased gall release to the intestines → emulsifies the lipids so they get absorbed.
• Bile salts: Inhibit synthesis of bile acids from cholesterol, but increases production of bile (choleretic effect)
• Gall bladder: Not present in horse, giraffe, camel, elephant, pigeon and rat. The continuously produced liver-bile is stored here, and extensively concentrated (due to absorption of Na+, Cl- and HCO3-). Results in bladder bile, which contains the concentrated bile components.