Bilirubin is formed as a result of the breakdown of hemoglobin from erythrocytes in the reticuloendothelial system. It becomes bound to albumin and transported through the blood to the liver. This free or unconjugated bilirubin is insoluble in water and cannot be filtered through the glomerulus of the kidney.

In the liver, bilirubin becomes conjugated with glucuronic acid to form bilirubin diglucuronide. This conjugated bilirubin, which is also called direct bilirubin, is water soluble and is excreted by the liver through the bile duct and into the duodenum.