Erythrocyte deformability refers to the ability of erythrocytes (red blood cells, RBC) to change shape under a given level of applied stress, without hemolysing (rupturing). This is an important property because erythrocytes must change their shape extensively under the influence of mechanical forces in fluid flow or while passing through microcirculation. The extent and geometry of this shape change can be affected by the mechanical properties of the erythrocytes, the magnitude of the applied forces, and the orientation of erythrocytes with the applied forces. Deformability is an intrinsic cellular property of erythrocytes determined by geometric and material properties of the cell membrane, although as with many measurable properties the ambient conditions may also be relevant factors in any given measurement. No other cells of mammalian organisms have deformability comparable with erythrocytes; furthermore, non-mammalian erythrocytes are not deformable to an extent comparable with mammalian erythrocytes.