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The principal determinants of blood viscosity are hematocrit, red blood cell deformability (i.e., the structural response of erythrocytes and their membranes to applied forces), and the viscosity of plasma. Of these, hematocrit has the greatest effect on blood viscosity: a 10% increase of this parameter typically increases viscosity at high shear rates by about 20%.1 Secondly, erythrocyte deformability also has an important effect on blood viscosity because the shape and velocity of red cells change substantially as they move from large to small vessels 2, and their deformability, which decreases as the cells age, can be a critical factor in their penetration into capillary vessels.
Plasma viscosity, the third major determinant of blood viscosity and is independent of the blood shear rate, but varies with concentrations of fibrinogen and other plasma proteins such as immunoglobulins and lipoproteins (chylomicrons, very-low density lipoprotein (VLDL) and low-density lipoprotein (LDL)). Other blood constituents including lipids, carbohydrates, leukocytes, and platelets also have direct effects on blood viscosity.3,4
1. Brun JF, Bouchahda C, Chaze D, Benhaddad AA, Micallef JP, Mercier J. The paradox of hematocrit in exercise physiology: which is the "normal" range from an hemorheologist's viewpoint? Clin Hemorheol Microcirc 2000;22:287-303.
2. Gaehtgens P, Pries AR, Ley K. Structural hemodynamic and rheological characteristics of blood flow in the circulation. In: Chien S, Dormandy J, Ernst E, Matrai A. Clinical hemorheology: applications in cardiovascular and haematological disease, diabetes, surgery, and gynecology. Boston: Martinus Niihoff; 1987. pp. 97-124.