Blood as a near-"ideal" emulsion: a retrospective on the concept of the red cell as a fluid drop, its implications for the structure of the red cell membrane.

Although the question whether the red cell is fluid or solid has been discussed since 17th century, it was the author's measurement of the relative viscosity of blood in 1960's that supplied the first direct evidence that the red cell interior is fluid. Furthermore, through his application of the equations of Taylor and, later, Oldroyd, to this problem, it became evident that, for the red cell to exhibit fluid-drop-like behavior, the membrane must also be fluid. This led to his concept of the red cell membrane as a complex two-phase structure (lipoprotein micelles and two-dimensional protein networks) which was similar to the one accepted nearly a decade later. The requirements of the theory of ideal emulsions that the shear stress be transmitted into the cell interior via low viscosity membrane, are met in the later work of other investigators using the concept of a tank-treading membrane having viscoelastic properties. This paper reviews the original work of the author which led to the development of an equation for the relative viscosity of blood as a function of volume concentration, C: nr = (1 - TkC)-2.5, valid at shear rates above 180 sec-1, in which T is the Taylor factor which gives a measure of fluidity of the red cell, and k is a plasma trapping factor. Both T and k increase with increasing rigidity of the red cell. Finally, the effect of the membrane viewed as a complex two-phase fluid, on the rheology of the red cell is discussed.