Influence of Deformability of Human Red Cells upon Blood Viscosity

<jats:p>The viscosity of blood at high rates of shear is unusually low compared to other suspensions of similar concentration. The underlying mechanisms were studied by rotational viscometry, red cell filtration, viscometry of packed cells and direct microscopic observation of red cells under flow in a transparent cone plate viscometer. Deformability of red cells was altered osmotically or abolished by aldehyde fixation. The normal red cells under isosmotic conditions passed easily through filter pores (5 to 14 µ diameter). After osmotic crenation, deformability of cells in pore flow was reduced. Normal cells were deformed into a variety of shapes at high rates of shear, while crenated cells tumbled undeformed. Suspensions of these normal cells showed more pronounced shear thinning (reduction of viscosity with increasing shear rate) than suspensions of crenated cells. Suspensions of rigid cells showed greatly increased viscosity and a shear thickening as a function of shear rate and shear time. The physiological deformability is of critical importance to blood flow at high rates of shear. This is possible through a fluid transition of the erythrocyte caused by a rotation of the membrane with and around the cell contents. This phenomenon is the prime cause of the progressive reduction in viscosity with increasing shear.</jats:p>