Schematic drawing of the different regimes of motion exhibited by RBCs in shear flow. (a) Tumbling at low shear stresses. The cell does not deform and rotates like a rigid body with a and b indicating the projection of the elliptical half axes in the paper plane. (b) At higher shear stress, the cell reorients itself and adopts the rolling motion. In this state of motion, the entire cell rotates. (c) At even higher shear stress, the RBCs reorient again and adopt the tank-treading state of motion. The cell is stretched and has a constant inclination angle θ with respect to the flow while the membrane rotates around the cytoplasm.