On the effect of dynamic flow conditions on blood microstructure investigated with optical shearing microscopy and rheometry.

Red blood cell (RBC) aggregation affects significantly the flow of blood at low shear rates. Increased RBC aggregation is associated with various pathological conditions; hence an accurate quantification and better understanding of the phenomenon is important. The present study aims to improve understanding of the effect of dynamic flow conditions on aggregate formation; whole blood samples from healthy volunteers, adjusted at 0.45 haematocrit were tested in different flow conditions with a plate-plate optical shearing system, image analysis, and a double-walled Couette rheometric cell. Results are presented in terms of aggregation index Aa, aggregate size index As and number of aggregates, which are shown to vary with shear rate gamma and with different shear rate variations with time gamma. The aggregation index Aa was observed to increase as the shear rate decreased between 10 and 3 s(-1). Above 10 s(-1), Aa was found to have a minimum value indicating minimal aggregation while, at approximately 3 s(-1), Aa reaches a maximum. The aggregation size index As, the number of aggregates, and the blood viscosity were found to vary considerably when the same sample was examined over the same shear rate range, but for different variations of shear rate with time, gamma.