Influence of flow properties of blood upon viscosity-hematocrit relationships.

The influence of the red cell concentration (per cent hematocrit) upon blood viscosity is well known. Some physiology texts (1) portray this relationship by means of a diagram in which relative or apparent viscosity is plotted against the per cent hematocrit (Figure 1). It has been demonstrated that besides the effect of cell concentration the viscosity of blood varies both as a function of the shearing stresses developed within the fluid and the rate at which these forces are distributed between the adjacent fluid elements during flow (2-4). The relationship or ratio of shear stress to the gradient of velocity, or shear rate, defines the viscosity of the fluid in the absolute dimensions of dyne-seconds-cm', or poise. In his original definition of this property of fluids, Newton made the assumption that the ratio of shear stress to shear rate was constant for all fluids. Contemporary studies of the flow properties of various complex fluids have shown that many of these demonstrate a disproportionate change in shear stress as shear rate increases or decreases; in fact, rheological nomenclature is based upon the direction in which this ratio changes with change in shear rate (5, 6). The viscosity of blood has been shown to exhibit this type of shear rate dependence, i.e., viscosity decreasing as shear rate rises (4). At the time Poiseuille made his classic contribution to the dynamics of flow in tubes, it was not known that complex fluids such as blood rarely maintained a constant of proportionality between shear stress and shear rate. The Poiseuillian equation therefore has as its first condition that the fluid under study be a Newtonian fluid, i.e., one with a constant ratio of shear stress to shear rate. Many workers have pointed out that blood is a non-Newtonian fluid (3, 7, 8), that the law of Poiseuille cannot be directly applied to the conditions of flow in the capillary circulation (9, 10), and that the viscosity of blood is anomalous (11-13). In all of these references , however, capillary tube viscometers or the vessels of experimental animals were used as the testing devices. The use of a capillary viscometer, which so well imitates the anatomy of a blood vessel, has certain practical limitations that make it difficult to derive values of blood viscosity over the lower ranges of shear rate (below 100 sec') (14, 15). The interpretation of the results of viscometry of such …