Involved in oxygen transport from the lung to the various peripheral tissues. Heterotetramer of two alpha-D chains and two beta chains. The component D forms dimers of tetramers upon deoxygenation. 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. Ref.6 chain was isolated from Hbm, the least abundant of the four early chick hemoglobins. Belongs to the globin family.