Figure 2.

Erythrocyte structure and content. One single erythrocyte contains about 250 million haemoglobin molecules. Haemoglobin, the protein responsible for oxygen transport, is a tetramer composed of 4 globin chains. Each globin chain contains one heme molecule responsible for the binding of one oxygen molecule (O₂). Heme consists of a porphyrin ring centred by one atom of iron (Fe²⁺). Oxygen strongly binds to this iron atom. Almost two-thirds of the body iron (about 2.5 to 36 g) is localised in haemoglobin of circulating erythrocytes.