Bilirubin is a bile pigment produced by the breakdown of haem and reduction of biliverdin. Unconjugated bilirubin is insoluble in plasma unless bound to protein, mainly albumin. Salicylates, sulphonamides, non-esterified fatty acids and reduced pH levels result in decreased protein-binding of unconjugated bilirubin. Normally, 95% of the circulating bilirubin is unconjugated.
The bilirubin-albumin complex is dissociated by receptors on hepatocytes. The albumin remains in the plasma. The bilirubin is taken into the hepatocyte and conjugated by the enzyme bilirubin UDP-glucuronyl transferase to form bilirubin diglucuronide. It is this water-soluble glucuronate derivative which is excreted into the biliary system.
In the gut, principally the colon, bilirubin glucoronides are degraded by bacteria and converted into a mixture of compounds, known as urobilinogen or stercobilinogen; these are water soluble.
Most of the urobilinogen is excreted in the faeces where it is oxidised to urobilin which is brown. Some is reabsorbed into the liver where it is re-excreted. When the amount of urobilinogen is increased, some passes into the systemic circulation and is excreted in the urine.
Complete biliary obstruction is indicated by:
References
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