The internal viscosity of the human erythrocyte may determine its lifespan in vivo.

Young red cells obtained by high-speed centrifugation of a normal blood sample had an MCHC of 31.7 g/dl which corresponds to an internal viscocity of about 9 cP. Old red cells from the same blood sample had an average MCHC of 37.5 g/dl which is equivalent to a mean internal viscosity of about 54 cP. This increased internal viscocity of old cells must result in a prolonged transit time through the narrow channels of the splenic microvasculature and consequently an increased probability of contact with a phagocytic macrophage. This probability of contact increases as the internal viscosity of the cell is further increased and may represent one of the major factors associated with the detection and subsequent elimination of senescent cells by the spleen.