Bilirubin is a linear tetrapyrrole. It is a product of the breakdown of circulating mature red blood cells in Kupffer cells of the liver and in the reticuloendothelial system. Immature and defective red cells are destroyed in the bone marrow. The average rate of production in the adult is 4 mg/kg per 24 hours, about 250-400 mg being produced daily. Eighty per cent of the total production of bilirubin is derived from the haem moiety of haemoglobin. The remainder of the bilirubin (shunt bilirubin) is produced from the catabolism in the liver of other haem-containing proteins such as myoglobin, cytochromes and catalases.
Increases in circulating bilirubin may arise from increased production, impaired hepatocyte uptake, defective conjugation in the hepatocyte and interference with the binding of bilirubin by albumin.
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