Metabolism of Bilirubin

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Bilirubin is a yellowish pigment that is formed during the breakdown of hemoglobin, a component of red blood cells. The metabolism of bilirubin primarily occurs in the liver and involves several steps:

  1. Hemolysis:

    • Bilirubin is derived from the breakdown of heme, a component of hemoglobin. When red blood cells reach the end of their lifespan (usually about 120 days), they are broken down in a process called hemolysis.

  2. Unconjugated Bilirubin:

    • The heme released during hemolysis is converted into unconjugated bilirubin, also known as indirect bilirubin. Unconjugated bilirubin is insoluble in water and cannot be easily excreted in the urine.

  3. Transport to the Liver:

    • Unconjugated bilirubin binds to albumin and is transported through the bloodstream to the liver.

  4. Uptake by Hepatocytes:

    • In the liver, unconjugated bilirubin is taken up by hepatocytes (liver cells) through active transport.

  5. Conjugation:

    • Within the hepatocytes, unconjugated bilirubin is conjugated with glucuronic acid, a process facilitated by the enzyme uridine diphosphate glucuronosyltransferase (UGT). This conjugation makes bilirubin water-soluble and able to be excreted in bile.

  6. Conjugated Bilirubin:

    • Conjugated bilirubin, also known as direct bilirubin, is water-soluble and can be excreted into the bile canaliculi.

  7. Excretion in Bile:

    • Conjugated bilirubin is transported through the bile ducts into the gallbladder and stored until needed. When bile is released into the small intestine, bilirubin is excreted in the feces.

  8. Conversion in the Intestine:

    • In the intestine, bilirubin undergoes further chemical changes by the action of bacteria. This leads to the formation of urobilinogen, some of which is reabsorbed into the bloodstream and excreted in the urine, giving it a yellow color.

  9. Fecal Excretion:

    • The remainder of urobilinogen is excreted in the feces, contributing to the brown color of stool.

Disruptions in any of these steps can lead to elevated bilirubin levels in the blood, resulting in jaundice (yellowing of the skin and eyes). Conditions such as liver disease, hemolysis, or bile duct obstruction can affect the metabolism of bilirubin. Monitoring bilirubin levels through liver function tests is crucial in assessing liver health and diagnosing related disorders.



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