The oxygen buffer function of hemoglobin is related to

Acid and base shifts in the blood are largely controlled by three main buffers in blood: (1) proteins, (2) hemoglobin, and (3) the carbonic acid-bicarbonate system. Plasma proteins are effective buffers because both their free carboxyl and their free amino groups dissociate. The second buffer system is provided by the dissociation of the imidazole groups of the histidine residues in hemoglobin. In the pH 7.0-7.7 range, the free carboxyl and amino groups of hemoglobin contribute relatively little to its buffering capacity. However, the hemoglobin molecule contains 38 histidine residues, and on this basis--plus the fact that hemoglobin is present in large amounts--the hemoglobin in blood has six times the buffering capacity of the plasma proteins. In addition, the action of hemoglobin is unique because the imidazole groups of deoxyhemoglobin (Hb) dissociate less than those of oxyhemoglobin (HbO2), making Hb a weaker acid and therefore a better buffer than HBO.Ref: Ganong's Review of Medical Physiology Twenty-Third Edition Page no: 614