Bilirubin

See also:

Bilirubin in lizards

Description

Bilirubin is a pigment formed from the haem groups of haemoglobin molecules. The enzymatic conversion occurs within cells of the monocyte-phagocyte system (MPS) when haemoglobin is released by the degradation of red blood cells. This unconjugated bilirubin is not water soluble and it is transported in plasma bound to albumin. It is taken up into hepatocytes where it is conjugated with glucuronic acid. The resultant conjugated bilirubin is water soluble and it 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. The latter alteration in faecal colour also results from steatorrhoea.

Small quantities of conjugated bilirubin are found in the urine of normal dogs because it has a low renal threshold.

Clinical Significance

Icterus refers to the staining of tissues by bilirubin pigment or bilirubin complexes, a phenomenon that is clinically evident on examination of the sclera and mucous membranes.