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Normal red cells deform markedly as they pass through the spleen and the peripheral capillaries. In these studies, the effects of Plasmodium falciparum infection and maturation on the deformability of parasitized red cells exposed to fluid shear stress in vitro were examined by means of a rheoscope. Red cells containing the early (ring) erythrocytic stage of the parasite have impaired deformability at physiologic shear stresses, and recover their normal shape more slowly. Red cells containing more mature parasites (trophozoites or schizonts) exhibit no deformation under the same conditions. These results provide a mechanism to explain the ability of the spleen to remove parasitized red cells from the circulation of both immune and nonimmune hosts.

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Measurements of the dimensions and membrane rotational frequency of individual erythrocytes steadily tank-treading in a rheoscope are used to deduce the surface shear viscosity of the membrane. The method is based on an integral energy principle which says that the power supplied to the tank-treading cell by the suspending fluid is equal to the rate at which energy is dissipated by viscous action in the membrane and cytoplasm. The integrals involved are formulated with the aid of an idealized mathematical model of the tank-treading red blood cell (RBC) (Keller and Skalak, 1982, J. Fluid Mech., 120:24-27) and evaluated numerically. The outcome is a surface-averaged value of membrane viscosity which is representative of a finite interval of membrane shear rate. The numerical values computed show a clear shear-thinning characteristic as well as a significant augmentation of viscosity with cell age and tend toward agreement with those determined for the rapid phase of shape recovery in micropipettes (Chien, S., K.-L. P. Sung, R. Skalak, S. Usami, and A. Tozeren, 1978, Biophys. J., 24:463-487). The computations also indicate that the rate of energy dissipation in the membrane is always substantially greater than that in the cytoplasm.

By means of glutaraldehyde fixation, human erythrocytes are "frozen" while suspended in turbulent shear flow. As the shearing is increased in steps from 100 to 2,500 dyn/cm2, the deformed cells evolve gradually toward a smooth ellipsoidal shape. At stresses above 2,500 dyn/cm2, approximately, fragmentation of the cells occurs with a concomitant increase in free hemoglobin content of the suspending medium. The photographic evidence suggests that the cells rupture in tension in the bulk flow.