Bilirubin

a yellow bile pigment derived from heme during the process of the breakdown of hemoglobin as erythrocytes are removed and destroyed by the reticuloendothelial system; constitutes between 40% and 95 % of the total yellow pigment in normal sera; found as sodium bilirubinate (soluble), or as an insoluble calcium salt in gallstones

Description
Conjugation of bilirubin with glucuronic acid occurs in the liver. Conjugated bilirubin is excreted from the liver cell into the bile canaliculus by an active process. In the intestinal tract a small fraction of the conjugated bilirubin excreted in the bile is hydrolysed and the unconjugated bilirubin reabsorbed. This is refered to as the enterohepatic circulation of bilirubin. The fate of most of the excreted bilirubin is conversion to urobilinogen by the flora of the large intestine. Excess bilirubin is associated with jaundice. In hemolytic disorders, ineffective erythropoiesis or Gilbert's syndrome, retention jaundice occurs, signifying the increased bilirubin above the capacity for hepatic removal and in that case unconjugated bilirubin is the predominant form of bilirubin presenti n serum, while in case of destruction or malfunction of hepatic parenchyma or obstruction to the outflow of bile, regurgitation jaundice occurs and in that case conjugatwed bilirubin is the predominant form in serum. In the laboratory the Van den Bergh test is used to differentiate between direct and indirect form of bilirubin in the serum (the direct bilirubin representing the conjugated with glucuronic acid form and the indirect representing the unconjugated form of bilirubin). Bilirubin in amniotic fluid is determined in order to evaluate hemolytic disease in the fetus in utero.