Explain the Enterohepatic Circulation of Bile
Any compound which is secreted in bile and subsequently reabsorbed from the small intestine, returns to the liver and is then resecreted in the bile. This is called enterohepatic circulation of bile. The enterohepatic circulation is a physiological conserving mechanism. The efficiency with which compounds recirculate may vary. Bile salts are efficiently reabsorbed. Only a minute proportion escapes into the systemic circulation. Cholesterol and phospholipids are much less efficiently absorbed. Let us study about the pathways of bile circulation. There is both a portal and extra portal pathway for the enterohepatic circulation. Bile acids undergo recirculation through the portal vein. Phosopholipids and cholesterol undergo an extra portal enterohepatic circulation. Various exogenous compounds such as antibiotics, barbiturates and digitalis undergo an enterohepatic circulation, which may be both portal and extraportal.
Compounds which undergo portal enterohepatic circulation are first secreted by the liver. Then they are passed into the intestine, then reabsorbed from the intestine and then transported back to the liver through the portal vein. Compounds with an extraportal enterohepatic circulation are absorbed from the small intestine into the lymphatics which then drain into superior vena cava. These compounds then enter the systemic circulation and are transported throughout the body where they may be partially excreted e.g. by kidney or by skin. They then pass back to the liver where they are re-excreted in bile.