Bilirubin levels

Bilirubin is the end product of the breakdown of hemoglobin from aged red blood cells. Bilirubin levels in the blood are an important indicator of liver function.

Bilirubin is formed after degradation of the heme group that is part of the hemoglobin protein found in red blood cells, and of other heme-containing proteins (such as cytochrome P450 enzymes, myoglobin and the enzyme catalase). It is formed in the macrophages of the spleen and bone marrow and in hepatic Kupffer cells, and released into the plasma.

As bilirubin is poorly soluble, much of it is bound to the protein albumin (known as free, unconjugated or indirect bilirubin) and when taken up by the liver it is conjugated with glucuronic acid and becomes more soluble. This form is known as conjugated or direct bilirubin. Finally, the conjugated form passes into the intestine through the bile where it is broken down by anaerobic bacteria into the pigment urobilinogen. Much of the urobilinogen is excreted with the feces and a smaller portion is reabsorbed in the intestine and eliminated in the urine.