Not true about bile salts
Correct Answer: Acts by increasing surface tension
Description: Bile is made up of the bile acids, bile pigments, and other substances dissolved in an alkaline electrolyte solution that resembles pancreatic juice. About 500 mL issecreted per day. Some of the components of the bile are reabsorbed in the intestine and then excreted again by the liver (enterohepatic circulation). The glucuronides of the bile pigments, bilirubin, and biliverdin, are responsible for the golden yellow color of bile. bile acids secreted into the bile are conjugated to glycineor taurine, a derivative of cysteine. The bile acids are synthesized from cholesterol. The two principal (primary) bile acids formed in the liver are cholic acid and chenodeoxycholic acid. In the colon, bacteria conve cholic acid to deoxycholic acid and chenodeoxycholic acid to lithocholic acid. In addition, small quantities of ursodeoxycholic acid are formed from chenodeoxycholic acid. Ursodeoxycholic acid is a tautomer of chenodeoxycholic acid at the 7-position. Because they are formed by bacterial action, deoxycholic, lithocholic, and ursodeoxycholic acids are called secondary bile acids. The bile salts have a number of impoant actions: theyreduce 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 thebile 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).REF: GANONG&;S REVIEW OF MEDICAL PHYSIOLOGY, KIM BARRETT, HEDDWEN BROOKS, SCOTT BOITANO, SUSAN BARMANTWENTY THIRD EDITIONPAGE NO:438,439,440
Category:
Physiology
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