Bilirubin metabolism

Bilirubin metabolism is the process by which the body breaks down and eliminates bilirubin, a yellow pigment produced from the breakdown of heme in red blood cells. Step 1: Heme Breakdown: Red blood cells have a lifespan of about 120 days. When they are old or damaged, they are broken down, primarily in the spleen, liver, and bone marrow. The heme component is converted into biliverdin, which is then reduced to bilirubin. Step 2: Unconjugated Bilirubin: This initial form of bilirubin is unconjugated (or indirect). It is lipid-soluble and not water-soluble, meaning it cannot be easily excreted by the kidneys. It circulates in the bloodstream bound to albumin, a protein. Step 3: Hepatic Uptake: In the liver, unconjugated bilirubin detaches from albumin and is taken up by liver cells (hepatocytes). Step 4: Conjugation: Inside the hepatocytes, unconjugated bilirubin is processed through a process called conjugation. It is combined with glucuronic acid by an enzyme called UDP-glucuronosyltransferase (UGT). This forms conjugated bilirubin (or direct bilirubin). Step 5: Excretion into Bile: Conjugated bilirubin is water-soluble and is actively transported from the hepatocytes into the bile ducts and then into the bile. Step 6: Intestinal Transit: Bile is released into the small intestine. In the intestine, bacteria further metabolize conjugated bilirubin into urobilinogen. Step 7: Elimination: Most urobilinogen is oxidized into stercobilin*, which gives feces its characteristic brown color and is eliminated in the stool. A small amount of urobilinogen is reabsorbed from the intestine back into the bloodstream and then excreted by the kidneys in the urine as urobilin*. This entire process efficiently removes bilirubin from the body. Problems at any stage can lead to jaundice.