Red cell physiology

The erythrocyte, the ultimate product of mammalian erythroid maturation, appears as a highly specialized and, paradoxically, a very simplified cell. In fact it lacks a nucleus and the intracellular organelles are essentially designed to transport oxygen from the outer environment to respiring tissues through the sophisticated functional properties of intra-erythrocytic hemoglobin. In this respect, since oxygen transport is such a vital process, the red cell has often been considered, in an excess of oversimplification, as merely a 'biological bag' enveloping a viscous solution of concentrated hemoglobin and containing only those few enzymes which are needed to maintain the cell functionally active. However, within the cell a number of different processes are contemporaneously going on, hemoglobin acts as an oxygen and carbon dioxide transporter, glycolysis and the pentose phosphate shunt are devoted to the production of ATP and NADPH, respectively, and membrane organization provides the cell with a good deformability, allowing it to cross narrow splenic capillaries and channels without any appreciable damage for several weeks of activity. All these processes are, indeed, highly integrated and concur to define a complex scenery centered on the oxygenation-deoxygenation cycle of hemoglobin. Within this emerging scheme, hemoglobin appears to display, besides the basic function of oxygen transport, several other biological functions which are driven by the oxygen-linked conformational transition and whose relative importance, in the economy of the cell and of the organism, is not easy to qualify. Some of these aspects are described and discussed.