Question

why is viscosity mainly in RBCs?

Answer 1

Hemorheology, also spelled haemorheology (from the Greek ‘αἷμα, haima "blood" and rheology), or blood rheology, is the study of flow properties of blood and its elements of plasma and cells. Proper tissue perfusion can occur only when blood's rheological properties are within certain levels. Alterations of these properties play significant roles in disease processes.[1] Blood viscosity is determined by plasma viscosity, hematocrit (volume fraction of red blood cell, which constitute 99.9% of the cellular elements) and mechanical properties of red blood cells. Red blood cells have unique mechanical behavior, which can be discussed under the terms erythrocyte deformability and erythrocyte aggregation.[2] Because of that, blood behaves as a non-Newtonian fluid. As such, the viscosity of blood varies with shear rate. Blood becomes less viscous at high shear rates like those experienced in peak-systole. Contrarily, during end-diastole, blood moves more slowly and becomes thicker and stickier. Therefore, blood is a shear-thinning fluid.

Answer 2

The effects of increased plasma viscosity and induced red blood cell (RBC) aggregation on apparent viscosity of blood in vivo in the skeletal muscle of the dog were studied. Apparent viscosity in vivo was determined in the isolated and vasodilated calf muscles of one hindlimb by comparing pressure-flow relationships for RBC suspensions with pressure-flow relationships for a Newtonian solution of known viscosity. RBC suspensions of increased plasma viscosity with and without RBC aggregation were obtained by substituting plasma with isoviscous solutions of high- and low-molecular-weight dextran in saline. Hematocrits of the suspensions were adjusted to either 45 or 60%. The viscosities of the suspensions in vitro were determined in a Wells-Brookfield viscometer. Apparent viscosity of blood in vivo was found to be mainly dependent on the viscosity of plasma. RBC aggregation had no significant influence on the viscosity in vivo.