Bilirubin

a yellow-brown bile pigment that gives the serum a yellow colour as a component of the blood. About 90% of it is formed as albumin-bound primary bilirubin during the oxidative degradation of haemoglobin in the →retikulohistiozytären system (especially in the spleen and liver) via its precursor →Biliverdin and from other sources. The primary water-insoluble bilirubin is synthesized in the liver cell into water-soluble secondary bilirubin (bilirubin conjugation) and secreted into the bile. After excretion into the duodenum, the back formation into unconjugated bilirubin and the gradual degradation (dehydrogenation) to urobilinogen/urobilinin, stercobilinogen/stercobilin and to pyrrole derivatives take place. Some of the bilirubin secreted in the intestine is not excreted in the stool, but reabsorbed, partly already in the duodenum, from where it returns to the liver in the portal vein blood and is excreted again (enterohepatic circulation), but partly only in the rectum, where it can be reabsorbed as already deconjugated bilirubin and reaches the kidney via the large circulation to be excreted with the urine.