Bilirubin is a byproduct of hemoglobin degradation. In humans, red blood cells are manufactured by the bone marrow. They have a circulating half life of approximately 120 days. After this time, the red blood cell membrane becomes less deformable. Red blood cells must be deformable in order to pass through the small blood vessels of the body(capillaries). If the red blood cell can no longer pass easily through these microscopic blood vessels, the red blood cell could then plug the blood vessel. This would prevent the delivery of oxygen to the cells surrounding these capillaries. Thus, the red blood cells need to be cleared from the circulation, and new blood cells take up the job of transport of oxygen. When this change in the red blood cell occurs (called cell membrane fluidity), the spleen removes the cell from the general circulation and begins to breakdown the components of the red blood cell. The spleen then degrades different portions of the red blood cell. Hemoglobin is then transferred to the liver. The liver then takes up hemoglobin and changes it into a variety of compounds called bilirubin. Bilirubin may be elevated due to a variety of abnormalities. These can be because of bile duct obstruction from any cause. In addition, it may be elevated because of diseases which are inherited such as Gilbert's disease or Crigler-Najjar Syndrome.
Bilirubin is composed of a direct (conjugated) or indirect (unconjugated) portions. A direct bilirubin elevation is usually a result of severe hepatocellular irritation or biliary tract obstruction.
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