Hemoglobin is the oxygen-carrying molecule of the red blood cells, which are essentially just tiny circulating bags of hemoglobin. This protein functions to contain iron ions in a certain way that allows them to reversibly bind oxygen molecules to themselves in the blood vessels of lungs and transport the oxygen molecules to the tissues of the body.

The fetus possesses a specialised form of hemoglobin known as hemoglobin F; this type binds and releases oxygen best under the conditions of low oxygen that are experienced in the womb. After the first few months of life hemoglobin F is almost totally replaced by the normal adult type hemoglobin A. Certain diseases are characterised by abnormal hemoglobin types, for example sickle cell disease. In this disease, an abnormal molecule known as hemoglobin S causes severe effects of capillary blood vessel blockage; another variant, hemoglobin C is common as well and causes similar disease as classic hemoglobin S sickling, but this condition is not as severe.

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