bilirubina

Bilirubina, commonly referred to as bilirubin, is a yellow-orange bile pigment produced during the breakdown of heme, a component of hemoglobin. It circulates in the blood mainly in an unconjugated form bound to albumin and is a major determinant of the color of bile and feces when processed by the liver and gut.

The production and metabolism of bilirubin involve several steps. Heme released from aging red blood cells is converted by heme oxygenase to biliverdin and then reduced to unconjugated bilirubin. This lipophilic molecule is transported to the liver bound to albumin. In hepatocytes, the enzyme system UDP-glucuronosyltransferase conjugates bilirubin with glucuronic acid to form conjugated bilirubin (bilirubin diglucuronide), which is water soluble and excreted into bile via canaliculi.

In the intestine, bacterial enzymes can deconjugate some bilirubin. Conjugated bilirubin is further reduced to urobilinogen; some of this is reabsorbed into the portal circulation and may be recycled (enterohepatic circulation). Most urobilinogen is converted to stercobilin, giving stool its brown color, while a portion is excreted in the urine as urobilin.

Clinical relevance and testing. Total bilirubin and its fractions (direct/conjugated vs indirect/unconjugated) help differentiate liver disease, biliary obstruction, and hemolysis. Conditions causing unconjugated hyperbilirubinemia include increased production or reduced conjugation (e.g., hemolysis, Gilbert syndrome). Conjugated hyperbilirubinemia points to intrahepatic or extrahepatic cholestasis. In newborns, physiologic jaundice results from immature conjugation, and severe cases require monitoring to prevent complications.