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The non-Newtonian flow properties of resuspended red cells were determined in vitro by means of a capillary viscometer. In order to evaluate the rheological effect of the suspending medium, viscosity measurements were made over a wide range of shearing stresses using both plasma and an acid-citrate-dextrose solution as diluents. At low shearing stresses, the plasma exhibited non-Newtonian flow behavior. Using a technique of treating the data to obtain rate-of-shear versus shearing-stress curves without prior assumption of a flow equation showed that whole blood over a wide range of shear stresses and a twofold range of capillary radii did not show any dependence of the viscosity on the capillaries employed. This procedure was also used to examine the data of other workers. In an attempt to determine the shape factor for the red cell, an extrapolation to infinite dilution and zero rate of shear was made. The shape factor can be estimated to be 2.5±1.5 for red cell.

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