Bilirubin Definition

A metabolic product of erythrocyte (red blood cell) heme that is a key component of hemoglobin. Bilirubin exists in two forms, conjugated (also called direct), which is water soluble, and unconjugated (also called indirect or free), which is fat soluble. The amounts and ratios of bilirubin present in the blood help doctors assess liver and gallbladder functions.

Normal Blood Bilirubin Values
unconjugated (indirect) bilirubin0.1 to 1.0 milligrams per deciliter (mg/dL)
conjugated (direct) bilirubin0.0 to 0.4 mg/dL
total bilirubin0.3 to 1.9 mg/dL

The spleen removes old erythrocytes (red blood cells), which contain high concentrations of hemoglobin, from the circulation and begins to break them down into their components. The bilirubin that results from this process is unconjugated, a form the body cannot eliminate. Albumin, a protein in the blood, transports this unconjugated bilirubin to the liver.

There, actions of an enzyme-glucuronyl transferase-help a chemical reaction that converts the unconjugated bilirubin to conjugated bilirubin, which then becomes an ingredient of bile.

Bilirubin is yellow and in turn colors the bile yellow; hence its designation as a bile pigment. Intestinal bacteria further metabolize bilirubin, the major component of which is urobilinogen. Urobilinogen gives the feces their characteristic dark color. Pale feces are a hallmark of disturbances of bilirubin metabolism.

Increased bilirubin levels in the blood result in jaundice, a yellowish discoloration of the skin most visible in the sclera (“white” of the eye). Certain wavelengths of light on the skin help the body complete bilirubin metabolism.


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