Bilirubin is absent in urine because it is:

Ans. is 'd' i.e. Lipophilic : Unconjugated bilirubin is absent in urine but conjugated bilirubin can be present in it. But I think that the examiner is talking about unconjugated bilirubin as 90% bilirubin in the serum is unconjugated.Unconjugated bilirubin is highly lipid soluble (Lipophilic) and has limited aqueous solubility thus it is not able to pass out in urine. Moreover unconjugated bilirubin is tightly bound to albumin and therefore not filtered by renal glomeruli.In contrast, conjugated bilirubin is water soluble, and easily filtered by the renal glomeruli..Metabolism of bilirubin:** Sources of bilirubinRBC's (old, senescent) 80%Ineffective erythropoiesisMetabolism of other heme containing proteinsThis unconjugated bilirubin is not water soluble so is bound to albumin for its transfer in plasmaLiver uptakes this unconjugated bilirubin and conjugates it with glucuronic acid, generating bilirubin glucuronide which is water soluble.Conjugated bilirubin (i.e. bilirubin glucuronide) is excreted in bile into the duodenum.Conjugated bilibrium is not reabsorbed by the intestinal mucosa. It is either excreted unchanged in the stools or metabolised by ileal and colonic bacteria to urobilinogen and related products.Urobilinogen can be reabsorbed from the small intestine and colon and enters the portal circulation. Some of the portal urobilinogen is taken up by the liver and reexcreted into the bile, and the remainder bypasses the liver and is excreted by the kidney.Urinary urobilinogen is a useful tool: for distinguishing possible causes of hyperbilirubinemia.Urinary urobilinogen excretion is increased in* :- * When production of bilirubin is greatly increased (eg. With hemolysis)** When the hepatic uptake and excretion of urobilinogen is impaired (eg in hepatocellular disease)*Urinary urobilinogen excretion is decreased in* :- * Cholestasis or extrahepatic biliary obstruction (Obstructive Jaundice)*