Bile salts are the sodium and potassium salts of bile acids, which are conjugated with glycine or taurine.
Formation of Bile Salts:
Bile salts are formed from bile acids. There are two primary bile acids in human, namely cholic acid and chenodeoxycholic acid, which are formed in liver and enter the intestine through bile. Due to the bacterial action in the intestine, the primary bile acids are converted into secondary bile acids:
- Cholic acid — deoxycholic acid
- Chenodeoxycholic acid —- lithocholic acid
Secondary bile acids from intestine are transported back to liver through enterohepatic circulation. In liver, the secondary bile acids are conjugated with glycine (amino acid) or taurin (derivative of an amino acid) and form conjugated bile acids, namely glycocholic acid and taurocholic acids. These bile acids combine with sodium or potassium ions to form the salts, sodium or potassium glycocholate and sodium or potassium taurocholate.
Enterohepatic Circulation of Bile Salts:
Enterohepatic circulation is the transport of substances from small intestine to liver through portal vein. About 90% to 95% of bile salts from intestine are transported to liver through enterohepatic circulation. Remaining 5% to 10% of the bile salts enter large intestine. Here, the bile salts are converted into deoxycholate and lithocholate, which are excreted in feces.
Functions of Bile Salts:
Bile salts are required for digestion and absorption of fats in the intestine. The functions of bile salts are:
1- Emulsification of Fats:
Emulsification is the process by which the fat globules are broken down into minute droplets and made in the form of a milky fluid catted emulsion in small intestine, by the action of bile salts. Lipolytic enzymes of Gl tract cannot digest the fats directly because the fats are insoluble in water due to the surface tension. Bile salts emulsify the fats by reducing the surface tension due to their detergent action. Now the fats can be easily digested by lipolytic enzymes.
Unemulsified fat usually passes through the intestine and then it is eliminated in feces. Emulsification of fats by bile salts needs the presence of lecithin from bile.
2- Absorption of Fats:
Bile salts help in the absorption of digested fats from intestine into blood. Bile salts combine with fats and make complexes of fats called micelles. The fats in the form of micelles can be absorbed easily.
3- Choleretic Action:
Bile salts stimulate the secretion of bile from liver. This action is called choleretic action.
4- Cholagogue Action:
Cholagogue acts as an agent that causes contraction of gallbladder and results in release of bile into the intestine. Bile salts act as cholagogues indirectly by stimulating he secretion of hormone cholecystokinin. This hormone causes contraction of gallbladder, resulting in release of bile.
5- Laxative Action:
Laxative is an agent which induces defecation. Bile salts act as laxatives by stimulating peristaltic movements of the intestine.
6- Prevention of Gallstone Formation:
Bile salts prevent the formation of gallstone by keeping the cholesterol and lecithin in solution. In the absence of bile salts, cholesterol precipitates along with lecithin and forms gallstone.