Erythrocyte margination and sedimentation in skeletal muscle venules

Previous studies in skeletal muscle of the dog and cat have shown that veno us vascular resistance changes inversely with blood flow and may be due mai nly to red blood cell aggregation, a phenomenon present in these species. T o determine whether red blood cell axial migration and sedimentation contri bute to this effect, we viewed either vertically or horizontally oriented v enules of the rat spinotrapezius muscle with a horizontally oriented micros cope during acute arterial pressure reduction. With normal (nonaggregating) rat blood, reduction of arterial pressure did not significantly change the relative diameter of the red blood cell column with respect to the venular wall. After induction of red blood cell aggregation in the rat by infusion of Dextran 500, red blood cell column diameter decreased up to 35% at low pseudoshear rates (below similar to5 s(-1)); the magnitude was independent of venular orientation. In vertically oriented venules, the plasma layer wa s symmetrical, whereas in horizontally oriented venules, the plasma layer f ormed near the upper wall. We conclude that, although red blood cell axial migration and sedimentation develop in vivo, they occur only for larger flo w reductions than are needed to elicit changes in venous resistance.