Bilirubin metabolism - Definition and Overview

Red blood cells (erythrocytes) are broken down in the spleen, liver sinusioids, and bones. The heme components are first converted to biliverdin, then to unconjugated bilirubin.

Because unconjugated bilirubin is insoluble, it binds to albumin for transport to the to the liver and then dissociates. The unconjugated bilirubin is then conjugated in the endoplasmic reticulum of the hepatocytes (liver cells).

Conjugated bilirubin, or bilirubinglucuronide, moves into the bile canaliculi of the liver and then to the gall bladder. When stimulated by eating, bile (including the conjugated bilirubin) is excreted into the small intestine.

In the later portions of the small intestine (ileum) and the colon, about half of the bilirubinglucuronide is converted into urobilinogen. Urobilinogen is either reabsorbed or converted by the presence of oxygen to sterobilin. The sterobilin and remaining bilirubinglucuronide are excreted in the feces. These two metabolites of bilirubin are what give feces their characteristic brown color.

Small amounts of urobilinogen remaining in the blood are filterd by the kidneys, ending up in the urine. This bilirubin metabolite gives urine its characteristic yellow color.