Effect of erythrocyte aggregation on velocity profiles in venules

A recent whole organ study in cat skeletal muscle showed that the increase in venous resistance seen at reduced arterial pressures is nearly abolished when the muscle is perfused with a nonaggregating red blood cell suspensio n. To explore a possible underlying mechanism, we tested the hypothesis tha t red blood cell aggregation alters flow patterns in vivo and leads to blun ted red blood cell velocity profiles at reduced shear rates. With the use o f fluorescently labeled red blood cells in tracer quantities and a video sy stem equipped with a gated image intensifier, we obtained velocity profiles in venous microvessels (45-75 mum) of rat spinotrapezius muscle at centerl ine velocities between 0.3 and 14 mm/s (pseudo-shear rates 3-120 s(-1)) und er normal (nonaggregating) conditions and after induction of red blood cell aggregation with Dextran 500. Profiles are nearly parabolic (Poiseuille fl ow) over this flow rate range in the absence of aggregation. When aggregati on is present, profiles are parabolic at high shear rates and become signif icantly blunted at pseudoshear rates of 40 s(-1) and below. These results i ndicate a possible mechanism for increased venous resistance at reduced flo ws.