Most potent stimulus for bile secretion
The bile salts have a number of impoant actions: they reduce surface tension and, in conjunction with phospholipids and monoglycerides, are responsible for the emulsification of fat preparatory to its digestion and absorption in the small intestine. They are amphipathic, that is, they have both hydrophilic and hydrophobic domains; one surface of the molecule is hydrophilic because the polar peptide bond and the carboxyl and hydroxyl groups are on that surface, whereas the other surface is hydrophobic. Therefore, the bile salts tend to form cylindrical disks called micelles. Their hydrophilic poions face out and their hydrophobic poions face in. Above a ceain concentration, called the critical micelle concentration, all bile salts added to a solution form micelles. Lipids collect in the micelles, with cholesterol in the hydrophobic center and amphipathic phospholipids and monoglycerides lined up with their hydrophilic heads on the outside and their hydrophobic tails in the center. The micelles play an impoant role in keeping lipids in solution and transpoing them to the brush border of the intestinal epithelial cells, where they are absorbed. Ninety to 95% of the bile salts are absorbed from the small intestine. Once they are deconjugated, they can be absorbed by nonionic diffusion, but most are absorbed in their conjugated forms from the terminal ileum (Figure 26-18) by an extremely efficient Na+-bile salt cotranspo system powered by basolateral Na+-K+ATPase. The remaining 5-10% of the bile salts enter the colon and are conveed to the salts of deoxycholic acid and lithocholic acid. Lithocholate is relatively insoluble and is mostly excreted in the stools; only 1% is absorbed. However, deoxycholate is absorbed. The absorbed bile salts are transpoed back to the liver in the poal vein and excreted in the bile (enterohepatic circulation). Those lost in the stool are replaced by synthesis in the liver; the normal rate of bile salt synthesis is 0.2 to 0.4 g/d. The total bile salt pool of approximately 3.5 g recycles repeatedly the enterohepatic circulation; it has been calculated that the entire pool recycles twice per meal and six to eight times per day. When bile is excluded from the intestine, up to 50% of ingested fat appears in the feces. A severe malabsorption of fat-soluble vitamins also results. When bile salt reabsorption is prevented by resection of the terminal ileum or by disease in this poion of the small intestine, the amount of fat in the stools is also increased because when the enterohepatic circulation is interrupted, the liver cannot increase the rate of bile salt production to a sufficient degree to compensate for the loss.REF: GANONG&;S REVIEW OF MEDICAL PHYSIOLOGY, KIM BARRETT, HEDDWEN BROOKS, SCOTT BOITANO, SUSAN BARMANTWENTY THIRD EDITIONPAGE NO:438,439,440
Cholecystokin
Secretin
Bile acid
Bile salt
G.I.T