Rheological effects of red blood cell aggregation in the venous network: Areview of recent studies

It has long been recognized that understanding the rheological properties o f blood is essential to a full understanding of the function of the circula tory system. Given the difficulty of obtaining carefully controlled measure ments in vivo, most of our current concepts of the flow behavior of blood i n vivo are based on its properties in vitro. Studies of blood rheology in r otational and tube viscometers have defined the basic properties of blood a nd pointed to certain features that may be especially significant for under standing in vivo function. At the same time, differences between in vivo an d in vitro systems combined with the complex rheological properties of bloo d make it difficult to predict in vivo blood rheology from in vitro studies . We have investigated certain flow properties of blood in vivo, using the venular network of skeletal muscle as our model system. In the presence of red blood cell aggregation, venous velocity profiles become blunted from th e parabolic as in Poiseuille flow, as pseudo-shear rate (= mean fluid veloc ity/vessel diameter) is decreased from similar to 100 s(-1) to 5 s(-1). At control flow rates, the short distance between venular junctions does not a ppear to permit significant axial migration and red cell depletion of the p eripheral fluid layer before additional red cells and aggregates are infuse d from a feeding tributary. Formation of a cell-free plasma layer at the ve ssel wall and sedimentation in vivo are evident only at very low pseudo-she ar rates (< 5 s(-1)). These findings may explain in large part observations in whole organs of increased venous resistance with reduction of blood flo w.