The cellular geometry, that is, the biconcave disc shape of red cells, is critical for the cells' survival. This cell surface shape provides a high ratio of surface area to cellular volume. The normal volume of the erythrocyte is approximately 90 m3. The minimum surface area that could encase this volume is a sphere of approximately 98 m3. The surface area of a biconcave disc enclosing this volume is approximately 140 m3. Thus, shape alone provides the red cell with a considerable amount of redundant membrane and cytoskeleton. This feature provides the extra membrane surface area needed when red cells swell. More importantly, this geometric arrangement allows red cells to stretch as they undergo deformation and distortion in response to the mechanical stress of the circulation. The consequent reduction in tolerance of these cells to osmotic stress explains why anaemias resulting from membrane defects often are accompanied by osmotic fragility, the basis for the clinical laboratory test. Similarly, if erythrocytes are engorged with water, they become macrospherocytic and less deformable (Dacie, J. V., Lewis, S. M., and Luzzatto, L., 1981).
Red Cell Membrane Permeability: The normal red cell membrane is nearly impermeable to monovalent and divalent cations, thereby maintaining a high potassium, low sodium, and very low calciu ...Show more