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[[File:Jaundiced cat.jpg|thumb|right|Image of a jaundiced cat, note the discoloured pinnae<br><small>(Image source Sabar 2007, Wikimedia Commons)</small>]]

[[File:Jaundiced cat.jpg|thumb|right|Image of a jaundiced cat, note the discoloured pinnae<br><small>(Image source Sabar 2007, Wikimedia Commons)</small>]]

Bilirubin is a pigment formed from the haem groups of haemoglobin molecules. [[Erythrocytes|Red blood cells]] either undergo phagocytosis in the case of ageing cells or haemolysis in haemolytic crises. Haemoglobin is freed from the red cells and is further broken down in the reticulo-endothelial system to haeme and globulin. Haeme is a mixture of iron and porphyrin. The enzymatic conversion occurs within cells of the monocyte-phagocyte system (MPS) when haemoglobin is released by the degradation of red blood cells.

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 indirect, in contrast to conjugated which can be referred to as direct bilirubin.

 

Both the iron and globulin are recycled for further use in [[erythropoiesis]]. The porphyrin from haemoglobin breakdown is converted to biliverdin, a green pigment, which may contribute to the greenish appearance seen in local bruising. Biliverdin is subsequently changed into bilirubin. This '''unconjugated''' bilirubin is not water soluble and is thus bound to albumin to be transported in the blood to the [[Liver - Anatomy & Physiology|liver]].

In the hepatocyte, bilirubin is released from the albumin and conjugated with glucuronic acid, forming water soluble '''conjugated''' bilirubin. This is secreted into bile which then moves into the small intestine.

 

Unconjugated bilirubin is sometimes referred to as indirect, in contrast to conjugated which can be referred to as direct 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 'alcoholic' 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 'alcoholic' colour due to the absence of stercobilin.  The latter alteration in faecal colour also results from steatorrhoea.