Bilirubin

When old erythrocytes are broken down in the reticuloendothelial system, bilirubin is produced. Heme is extracted from haemoglobin and other heme-containing proteins, converted to bilirubin, and then delivered to the liver in a combination with serum albumin. Bilirubin is conjugated with glucuronic acid in the liver to be solubilized, transported through the bile duct, and then eliminated through the gastrointestinal tract. The blood levels of unconjugated (indirect) bilirubin rise in diseases or disorders that, through hemolytic mechanisms, create bilirubin more quickly than the liver can metabolise it. The elevated levels of circulating unconjugated bilirubin are similar in conditions such as liver immaturity and a number of other disorders where the bilirubin conjugation pathway is compromised. Elevations of conjugated (direct) and unconjugated (indirect) bilirubin in the blood are brought on by hepatocellular injury or bile duct obstruction.