The oxygen-carrying pigment that is present in red blood cells. Haemoglobin molecules, which are produced by bone marrow, are made up of 4 protein chains (2 alpha- and 2 betaglobin) and 4 haem (a red pigment that contains iron). Oxygen from the lungs enters red blood cells in the bloodstream. The oxygen then combines chemically with the haem within the haemoglobin to form oxyhaemoglobin, which gives blood in the arteries its distinctive bright red colour and is carried around the body. In areas that need oxygen, the oxyhaemoglobin releases its oxygen and reverts to haemoglobin, giving blood in the veins its distinctive darker colour. Some defects in haemoglobin production result from a genetic disorder; such defects are subdivided into errors of haem production, known as porphyrias, and those of globin production, known as haemoglobinopathies. Other defects, such as some types of anaemia, have a nongenetic cause.


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