Contributions of collision rate and collision efficiency to erythrocyte aggregation in postcapillary venules at low flow rates

Abstract

Kim S, Zhen J, Popel AS, Intaglietta M, Johnson PC. Contributions of collision rate and collision efficiency to erythrocyte aggregation in postcapillary venules at low flow rates. Am J Physiol Heart Circ Physiol 293: H1947-H1954, 2007. First published July 6, 2007; doi:10.1152/ajpheart.00764.2006.-Red blood cell aggregation at low flow rates increases venous vascular resistance, but the process of aggregate formation in these vessels is not well understood. We previously reported that aggregate formation in postcapillary venules of the rat spinotrapezius muscle mainly occurs in a middle region between 15 and 30 m downstream from the entrance. In light of the findings in that study, the main purpose of this study was to test two hypotheses by measuring collision frequency along the length of the venules during low flow. We tested the hypothesis that aggregation rarely occurs in the initial 15-m region of the venule because collision frequency is very low. We found that collision frequency was lower than in other regions, but collision efficiency (the ratio of aggregate formation to collisions) was almost nil in this region, most likely because of entrance effects and time required for aggregation. Radial migration of red blood cells and Dextran 500 had no effect on collision frequency. We also tested the hypothesis that aggregation was reduced in the distal venule region because of the low aggregability of remaining nonaggregated cells. Our findings support this hypothesis, since a simple model based on the ratio of aggregatable to nonaggregatable red blood cells predicts the time course of collision efficiency in this region. Collision efficiency averaged 18% overall but varied from 0 to 52% and was highest in the middle region. We conclude that while collision frequency influences red blood cell aggregate formation in postcapillary venules, collision efficiency is more important. red blood cells; hemorheology; venous vascular resistance AT LOW SHEAR RATES, red blood cells in the blood of athletic species, but not sedentary species, tend to form clusters or linear stacks (rouleaux) (20). This feature has been attributed to a bridging phenomenon (7,14) or to an osmotic exclusion effect between adjacent cells by macromolecules (1, 19). Previous studies in this laboratory have shown that in the venular network, where shear rates are normally low, aggregation creates an inverse relation between flow rate and venous vascular resistance. The small venules contribute importantly to this relationship (10). This property may be important in maintaining a constant capillary hydrostatic pressure in skeletal muscle in athletic species where flow rate undergoes large changes (5). The aggregation also appears to influence the distribution of red blood cells in the capillary network by causing plasma skimming at bifurcations (12, 13) or by plugging capillaries and causing flow stasis at high levels of aggregation (16,17).While these previous studies indicate that aggregation is functionally important, relatively little is kno...

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