a chemical breakdown product of hemoglobin. Hemoglobin is a substance in red blood cells that grabs onto oxygen in the lungs and carries it to the tissues in the body where it releases it. When red blood cells wear out they are trapped in the spleen and destroyed, releasing bilirubin into the blood. This type of bilirubin is called unconjugated. The liver takes the bilirubin out of the blood and conjugates it by attaching a portion of another molecule to the bilirubin molecule. www.classkids.org