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==Description==
==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.
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. Unconjugated bilirubin is sometimes referred to as direct, in contrast to unconjugated which can be referred to as indirect bilirubin.
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.
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.