oxygen from the lungs to the tissues.
It is bright red when combined with oxygen and purple-blue in the deoxygenated state. Each molecule is made up of a globin (a type of protein) and four heme groups. Heme, a complex heterocyclic compound , is an organic molecule derived from porphyrin with an iron atom at the centre. Variant hemoglobins (see sickle-cell anemia ; hemoglobinopathy ) can be used to trace past human migrations and to study genetic relationships among populations.