Bilirubin

Bilirubin is a pigment of red blood cells which is released into plasma cells when these cells are degraded in the monocytic phagocyte system (MPS). This unconjugated bilirubin is transported in plasma bound to albumin and taken up into hepatocytes where it is conjugated with glucuronic acid. The resultant conjugated bilirubin is secreted into bile which then moves into the small intestine. The conjugated bilirubin is degraded to urobilinogen by gastro-intestinal bacteria and a small proportion of this product is reabsorbed and excreted in the urine. The remaining urobilinogen is further degraded to stercobilin, a brown pigment which contributes to the colour of faeces. Therefore, in animals with complete biliary obstruction, urobilinogen is absent from the urine and the faeces have a white/grey 'acholic' colour due to the absence of stercobilin. Small quantities of conjugated bilirubin are found in the urine of normal dogs because it has a low renal threshold.