||This article needs additional citations for verification. (July 2011)|
Enterohepatic circulation refers to the circulation of biliary acids, bilirubin, drugs, or other substances from the liver to the bile, followed by entry into the small intestine, absorption by the enterocyte and transport back to the liver.
Hepatocytes metabolize cholesterol to cholic acid and chenodeoxycholic acid. These lipid-soluble bile acids are conjugated mainly to glycine or taurine molecules to form water soluble primary conjugated bile acids, sometimes called "bile salts". These bile acids travel to the gall bladder during the interdigestive phase for storage and to the descending part of the duodenum via the common bile duct through the major duodenal papilla during digestion. 95% of the bile acids which are delivered to the duodenum will be recycled by the enterohepatic circulation.
Due to the pH of the small intestine, most of the bile acids are ionized and mostly occur as their sodium salts which are then called “primary conjugated bile salts.” In the lower small intestine and colon, bacteria dehydroxylate some of the primary bile salts to form secondary conjugated bile salts (which are still water soluble). Along the proximal and distal ileum, these conjugated primary bile salts are reabsorbed actively into hepatic portal circulation. Bacteria deconjugate some of the primary and secondary conjugated bile salts back to lipid soluble bile acids, which are passively absorbed into hepatic portal circulation. Finally, the conjugated bile acids which remained un-ionized conjugated bile acids are passively absorbed.
Venous blood from the ileum goes straight into the portal vein and then into the liver sinusoids. There, hepatocytes extract bile acids very efficiently, and little escapes the healthy liver into systemic circulation.
The net effect of enterohepatic recirculation is that each bile salt molecule is reused about 20 times, often multiple times during a single digestive phase.
Bilirubin is conjugated with glucuronic acid in the liver by the enzyme glucuronyltransferase, making it soluble in water. Much of it goes into the bile and thus out into the small intestine. However 95% of the secreted bile is reabsorbed by the small intestine. This bile is then resecreted by the liver into the small intestine. About half of the conjugated bilirubin remaining in the large intestine(about 5% of what was originally secreted) is metabolised by colonic bacteria to urobilinogen, which is then further oxidized to urobilin and stercobilin. Urobilin, stercobilin and their degradation products give feces its brown color.  However, just like bile, some of the urobilinogen is reabsorbed, and 95% of what is reabsorbed is resecreted in the bile which is also part of enterohepatic circulation. A small amount of the reabsorbed urobilinogen(about 5%) is excreted in the urine where it is converted to an oxidized form, urobilin, which gives urine its characteristic yellow color. This whole process results in only 1-20% of secreted bile being lost in the feces. The amount lost depends on the secretion rate of bile.
Enterohepatic circulation also means that some molecules which would not otherwise be very toxic can become extremely hepatotoxic as they reach unexpectedly high hepatic concentrations. Drugs may remain in the enterohepatic circulation for a prolonged period of time as a result of this recycling process.