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Blood is a suspension of cellular elements in plasma, therefore exhibit non-Newtonian flow behavior. That is, its viscosity is shear rate dependent. Blood viscosity decrease with increased shear rate, known as shear thinning.

Blood viscosity is determined by plasma viscosity, hematocrit (volume fraction of red blood cell, which constitute 99.9% of the cellular elements) and mechanical behavior of red blood cells. Therefore, red blood cell mechanics is the major determinant of flow properties of blood. Red blood cells have unique mechanical behavior, which can be discussed under the terms “erythrocyte deformability ” and “erythrocyte aggregation ”.[2]

Blood viscosity can be measured by viscometers capable of measurements at various shear rates, such as a rotational viscometer.[3]