Non‐Newtonian Rheology of Human Blood ‐ Effect of Fibrinogen Deduced by “Subtraction”

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.