Transmission electron micrograph showing biconcave disc shape of normal red blood cells

The biconcave disc shape of the RBC is determined by the ratio of membrane surface area to internal cytoplasmic volume, referred to as the surface area-to-volume ratio (SA/V). The membrane surface area in turn depends on a highly regulated array of membrane phospholipids in a lipid bilayer and an underlying cytoskeletal network, which are linked via vertical associations with integral membrane proteins. Refer to UpToDate discussions of the RBC membrane for details.