Enterohepatic Circulation - Bilirubin

Bilirubin

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.