No abstract

Static normal human blood possesses a distinctive yield stress. When the yield stress is exceeded, the same blood has a stress-shear rate function under creeping flow conditions closely following Casson's model, which implies reversible aggregation of red cells in rouleaux and flow dominated by movement of rouleaux. The yield stress is essentially independent of temperature and its cube root varies linearly with hematocrit value. The dynamic rheological properties in the creeping flow range are such that the relative viscosity of blood to water is almost independent of temperature. Questions raised by these data are discussed, including red cell aggregation promoted by elements in the plasma.

A study of the rheological properties of human blood, from donors in normal health, was carried out by means of a coaxial cylinder viscometer designed to measure very small levels of stress under conditions of "creeping" flow. It was found that under these conditions of measurement the rheological properties could be conveniently presented by plotting the square root of shear stress against square root of shear rate. For normal blood, a nearly linear relation is found on such a plot, and the intercept on the stress axis at zero shear rate represents the square root of yield stress, separate determination of which is made by other means. Similar plots for (i) defibrinated blood and (ii) suspensions of red cells in isotonic saline solution reveal no yield stress. Thus it is concluded that fibrinogen is essential for the existence of yield stress in human blood. Furthermore, the approximate linearity, for normal blood, of the square root of shear stress with square root of shear rate, and the yield stress intercept, are of great interest inasmuch as mathematically identical relations ensue according to an equation developed by Casson based on a physical model in which the elementary particles of a suspension are capable of reversible association into rod-like structures, the length of which is controlled by the shear rate. It is of interest to consider the Casson model in the light of rouleaux formation and the relation of fibrinogen to rouleaux formation.

Rheological properties of normal human blood as a homogeneous substance: its yield stress and the stress-shear rate function under creeping shear rates, were directly measured for the first time in a Couette type viscometer, the cylinders of which were grooved to minimize heterophase effects (defined herein). The experimental data are well fitted by the Casson equation, which implies reversible aggregation of red cells into rouleaux at low shear rates. In particular, the value of yield stress extrapolated from the Casson equation is found to be equal to the directly measured yield stress within ± 10%. The yield stress is almost independent of temperature over the range 10–37 C. The following questions related to flow in the microcirculation are considered: the relation of yield stress to pressure drop and to the critical closing pressure, the significance of the Casson model to slow flow, and use of rheological parameters in general for microcirculatory flow calculations. Note: (With the collaboration of C. S. Draper, P. J. Gilinson, Jr., C. R. Dauwalter, M. Grove-Rasmussen, and R. Shaw) Submitted on July 10, 1962