Involved in oxygen transport from the lung to the various peripheral tissues. There are three forms of hemoglobin in Sphenodon: A, A' and D. Hb A is a tetramer of two alpha-A and two beta-1, Hb A' is a tetramer of two alpha-a and two beta-2, Hb D is a tetramer of two alpha-D and two beta-2. Sphenodon Hbs have properties not found in other reptiles: poor cooperativity, high affinity for oxygen, small Bohr and haldane effects, appreciable phosphate effects (those properties are also found in the Hbs of primitive urodele and caecilian amphibians). Belongs to the globin family.