Fast response characteristics of red blood cell aggregation.

The present work reports on an important feature of the fast response dynamics of blood flow observed after abrupt changes of the shearing conditions: distinctive peak values in conductance and light reflection/transmission have been observed at short times after the abrupt changes in the shearing conditions and have been attributed to red blood cell (RBC) disorientation and shape changes. Optical shearing microscopy results from the present study show that this peak is directly related to the inter-cellular or inter-aggregate spacing, quantified as the plasma gaps present in the captured images. In order to provide a more in-depth understanding of the structural characteristics of blood subjected to abrupt changes in the flow conditions, normal human blood samples at hematocrits of 45, 35, 25 and 10% were sheared at 100 s(-1) and the shear then suddenly reduced to values decreasing from 60 to 0 s(-1). Results from the present study agree qualitatively and quantitatively with results previously reported in the literature: the hematocrit and the magnitude of the final shear rate affect the magnitude of the peak values. The characteristic peak time was mostly influenced by the cell concentration. It is suggested that aggregation forces may play a part in the process of the fast response structural and spatial rearrangements of RBC.

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.