Red cell velocity during functional hyperemia: implications for rheology and oxygen transport.

Muscle blood flow increases during work. Any associated change in blood velocity that occurs during functional hyperemia can have profound effects on wall shear rate and arteriolar hemoglobin saturations. We measured arteriolar red cell velocity and cross-sectional area during muscle contraction to determine the physiological significance of any of these changes in calculations of wall shear rate and the in situ spectrophotometric measurement of hemoglobin oxygen saturation. Calculated cremaster muscle blood flow increased 64-236% during twitch and tetanic stimulation, respectively, which was due entirely to an increase in cross-sectional area, with muscle work producing little change in either the red cell velocity or the calculated wall shear rate. Small changes ranging from a 3% increase to a 4% decrease in hemoglobin saturation were evident in second- and third-order arterioles, which apparently reflects offsetting effects of the increase in metabolic rate and the increase in arteriolar blood volume. A simple model explaining the microcirculatory adjustments made during muscle work requires dilation of both feed arteries and arterioles if red cell velocity is to remain constant during hyperemia.