Rheology in the microcirculation in normal and low flow states.

A brief review is given for the rheological behavior of blood in the microcirculation in normal and low flow states. Blood viscosity increases in low flow conditions because of the decrease in shear stress which normally causes red cell deformation and disaggregation. The low hematocrit found in the microvessels leads to a decrease in apparent blood viscosity and tends to compensate for the high vascular hindrance due to the narrowness of the vessels. The presence of the less deformable white blood cells in the microcirculation causes a retardation of red cell flux and a redistribution of red cells at bifurcations. Elevated hematocrit, in turn, can cause an enhancement of the interaction between white blood cells and the venular endothelium. Adhesion of white blood cells to microvascular endothelium leads to an increase in flow resistance, especially in low flow states. The optimum hematocrit for myocardial oxygen transport and utilization is shifted from near 40% in normal conditions to 25% in hemorrhagic hypotension. These findings indicate that blood rheology in the microcirculation should be considered in analyzing the pathophysiological disturbances in shock and that improvement of microcirculation rheology offers a promising approach in treatment.