Study of the effect of varying hematocrit on free deformation and orientation of erythrocytes in flow.

The electron paramagnetic resonance (EPR) spin label method was used to investigate the effect of varying hematocrit on the deformation and orientation behavior of erythrocytes in shear flow. The relative EPR spectral change due to flow, which we use as a measure of the average deformation and orientation of erythrocytes, was observed as a function of the hematocrit. The profile generally shows a rising and a declining phase with the maximum in-between. The position of the maximum with respect to the hematocrit and the level of the spectral change are influenced by the suspending medium viscosity, osmolarity, and depend upon modifications of the red cell properties such as the internal viscosity, area-to-volume ratio and membrane rigidity. Results show that there is an upper limit of free deformation and orientation of the cell for a given hematocrit value. A possible role of the cell-cell interaction is discussed in restricting the space around a cell which is required for free deformation and orientation. A significance of the findings is that the actual deformation and orientation of an ensemble of cells which give rise to the EPR spectral change in flow is determined not only by the single cell deformability but also by the way the cells interact with each other under a given fluid dynamic condition.