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Haemoglobin is a complex molecule contained within erythrocytes that binds to and transports oxygen and carbon dioxide around the body.

Each haemoglobin molecule has four haem groups and thus can bind four oxygen molecules. Hb becomes saturated with oxygen in the pulmonary capillaries where pO2 is high and there is a high affinity for O2. Each gram of haemoglobin can carry approximately 1.4 ml of oxygen.

Note that the iron in haem is in the Fe2+ form, and that this does not change with oxygenation. The Fe3+ form is associated with methaemoglobinaemia.

The biconcave shape of the red cell provides a large surface area for the uptake and release of O2 and CO2.

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