Involved in oxygen transport from the lung to the various peripheral tissues. Heterotetramer of two alpha-D chains and two beta chains. Red blood cells. In birds, the alpha-D chain occurs in a minor hemoglobin component, called hemoglobin d, which is expressed in late embryonic and adult life. Belongs to the globin family. The authors believe the gene from which the sequence was translated to be functional although the second intron begins with 'GC' rather than the usual 'GT'.