The motion and shape of red blood cells depends on the flow environment. (Top) The shape of a red blood cell at rest is symmetric, with both sides of the membrane concave. (Bottom left) The velocity profile of the fluid that carries the cells through a blood vessel is assumed to have a symmetric parabolic shape. (Bottom right) The simulated shapes of the cells as a function of the surrounding flow velocity and their reduced area , which is the ratio of their area to the area of a circle with a circumference equal to the perimeter of the cell. Below a certain value of , the cells are unstable to forming an asymmetric shape (from Ref. ).