Red blood cell velocity profiles in skeletal muscle venules at low flow rates are described by the Casson model.

Knowledge of the effects of red blood cell aggregation on blood flow in small vessels is crucial to a better understanding of resistance changes in the venous microcirculation. Recent studies on rat spinotrapezius muscle indicate that enhanced red blood cell aggregation, induced by dextran 500, significantly affects velocity profiles at pseudoshear rates (the ratio of mean velocity to diameter) less than 40 s(-1). Since the use of a power-law model to describe these profiles does not provide a consistent rheological description, we have evaluated using the Casson model that has been widely used to characterize in vitro blood rheology. In the present study, we report experimental values of rat blood viscosity in the presence of dextran 500 and combine these in vitro measurements with previously obtained in vivo venular velocity profiles to determine whether the Casson model can provide a valid description of in vivo velocity profiles. Our analysis shows that the two-phase Casson model with a peripheral plasma layer is in quantitative agreement with experimentally obtained velocity profiles obtained in venules of rat spinotrapezius muscle under low flow rate. These results have implications for pathological low-flow conditions, such as hemorrhage and sepsis, and they quantitatively describe blunted velocity profiles and elevated flow resistance in postcapillary venules.