In the lungs, oxygen diffuses from alveolar air into the blood
because the venous blood has a lower partial pressure. The oxygen
dissolves in the blood. Only a small amount is carried as a physical
solution (0.31 ml per 100 ml). The remainder of the oxygen is
carried in chemical combination with the hemoglobin in red blood
Hemoglobin (molecular weight of 68,000) is made from 4 hemes,
a porphyrin ring containing iron and globin, a 4 protein chains.
Oxygen is bound to the iron for the transport process. Hemoglobin
(HHgb) behaves as a weak acid (K = 1.4 x 10-8; pKa
= 7.85). Oxyhemoglobin (HHgbO2) also behaves as a
weak acid (K = 2.5 x 10-7; pKa = 6.6)
Because both forms of hemoglobin are weak acids, and a relationship
of the numerical values of the equilibrium constants, the net
reaction for the interaction of oxygen with hemoglobin results
in the following equilibrium:
HHgb + O 2 <===> HgbO 2 + H+
If 2 is increased in the blood at the lungs, the
equilibrium shifts to
the right and H+ ions increase.
Oxyhemoglobin can be caused to release oxygen by the addition
of H+ ions at the cells. The difference in pH (7.44)
of arterial blood and venous blood (pH = 7.35) is sufficient
to cause release of oxygen from hemoglobin at the tissue cells.