Bilirubin is absent in urine because it is
Correct Answer: Not filterable
Description: Bilirubin metabolism and bile The liver plays a central role in the metabolism of bilirubin and is responsible for the production of bile (Fig. 22.6). Between 425 and 510 mmol (250-300 mg) of unconjugated bilirubin is produced from the catabolism of haem daily. Bilirubin in the blood is normally almost all unconjugated and, because it is not water-soluble, is bound to albumin and does not pass into the urine. Unconjugated bilirubin is taken up by hepatocytes at the sinusoidal membrane, where it is conjugated in the endoplasmic reticulum by UDP-glucuronyl transferase, producing bilirubin mono- and diglucuronide. Impaired conjugation by this enzyme is a cause of inherited hyperbilirubinaemias . These bilirubin conjugates are water-soluble and are expoed into the bile canaliculi by specific carriers on the hepatocyte membranes. The conjugated bilirubin is excreted in the bile and passes into the duodenal lumen. Once in the intestine, conjugated bilirubin is metabolised by colonic bacteria to form stercobilinogen, which may be fuher oxidised to stercobilin. Both stercobilinogen and stercobilin are then excreted in the stool, contributing to its brown colour. Biliary obstruction results in reduced stercobilinogen in the stool, and the stools become pale. A small amount of stercobilinogen (4 mg/day) is absorbed from the bowel, passes through the liver and is excreted in the urine, where it is known as urobilinogen or, following fuher oxidisation, urobilin. The liver secretes 1-2 L of bile daily. Bile contains bile acids (formed from cholesterol), phospholipids, bilirubin and cholesterol. Several biliary transpoer proteins have been identified . Mutations in genes encoding these proteins have been identified in inherited intrahepatic biliary diseases presenting in childhood, and in adult-onset disease such as intrahepatic cholestasis of pregnancy and gallstone formation. Urine tests If the blood level of conjugated bilirubin becomes elevated, e.g. due to liver disease, excess conjugated bilirubin is excreted in the urine, indicating a pathological process. Unconjugated bilirubin is not water-soluble and so is not excreted in the urine. Ref Davidson edition23rd pg 851
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