Three Rheologic States of Erythrocytes Depending Upon Increasing Shear Rate. Each state, tumbling (flipping), rolling (spinning), and tank-treading/swinging shows one erythrocyte with sequential changes in flow from left to right. In each rheologic state, the erythrocyte has a black dot representing an adherent microbead, as used experimentally to mark a location on the red cell membrane. At low shear rates on the left, the erythrocyte tumbles (flips) while maintaining membrane solidity so that the microbead remains in the same location relative to the rest of the cell. As the shear rate is increased (left green arrow), the orientation of the erythrocyte in the shear plane allows a wheel-like rolling, while the microbead remains in the same location because the membrane remains relatively solid. With further increases in shear rate, the rolling erythrocyte reorients itself by 90 degrees, so that it is spinning while maintaining membrane solidity. With even further increases in shear rate (right green arrow), the erythrocyte membrane fluidity reaches the point at which the membrane rotates around the cytoplasm (tank-treads), and the microbead moves relative to its former location on the erythrocyte. The tank-treading erythrocyte maintains its basic biconcave shape, but it has slight fluctuations in its orientation so that it “swings” relative to the shear plane.