How is bilirubin formed?

Bilirubin is formed from the breakdown of hemoglobin in red blood cells. When red blood cells reach the end of their lifespan, they are phagocytized by macrophages, which convert heme, a component of hemoglobin, into biliverdin. Biliverdin is then rapidly converted into bilirubin by the enzyme biliverdin reductase. This unconjugated bilirubin is transported to the liver, where it is conjugated with glucuronic acid for excretion.