• Bilirubin, the principle pigment in bile, is derived from the breakdown of hemoglobin

• About 85% of bilirubin produced in the body daily is released from hemoglobin as a result of RBCs destruction.

• The remaining 15% of bilirubin is produced from erythrocyte precursors destroyed in the bone marrow and from the catabolism of other heme containing proteins such as myoglobin, cytochromes and peroxidases.

• After it is produced in peripheral tissues, bilirubin is transported to the liver in association with albumin.

• Bilirubin is then rapidly taken up by hepatocytes across the sinusoidal membrane by a carrier mediated, active transport process.

• Once inside the liver cells, bilirubin is rapidly conjugated with glucuronic acid (glucuronyl transferase) to produce conjugated bilirubiun, which is then excreted into bile.

B- Excretion of bile:

• As already explained bile is composed of bile acids or salts, bile pigments(bilirubin esters), cholesterol and other substances excreted from the blood.
• Total bile production averages about 3 L/day but only 1 L is excreted, due toenterohepatic circulation.