Chemical structure of erythrocytes with emphasis on Donnan equilibrium.

Hemoglobin is a major constituent of erythrocytes. Because it is non-diffusable and confined within a cell membrane and because of its buffering powers, three major physiochemical equilibria are involved in any acid or base change in blood. Ionic, osmotic and Donnan balance must be main­ tained.16 During the acidosis phase of cell­ ular respiration and increasing Pco2, bi­ carbonate enters erythrocytes and replaces the basic anion form of hemoglobin which is in ionic balance, chiefly with K+. This replacement causes a slight hyperosmolarity which is corrected by a water shift from plasma to cells. The relative excess of bicarbonate within the cells to that out­ side the cells is corrected by an exchange with some of the plasma chloride ions. A similar chain of events occurs as metabolic acidosis develops, and an opposite situation occurs during the periods of developing alkalosis. When oxygen is taken up by blood in the lungs, the resulting oxyhemoglobin in­ crease is more acidic than the reduced hemoglobin form. This change aids in the freeing of C 0 2 from plasma bicarbonate much in the same manner that other acid compounds convert bicarbonate to C 0 2. At the tissue level, the reverse reaction aids uptake of C 0 2. This reaction is dependent upon both an immediate and a delayed acidosis.1 This paper deals mainly with these two physiological situations and the shifts which occur across the erythrocyte mem­ brane.