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Figure 1 The inverse relationship between viscosity and shear rate is shown for a standard Poiseuille flow curve. Shear is defined as (v₂ – v₁)/d, where (v₂ – v₁) is the difference in velocity between one flow layer and its adjacent layer and d is the distance between layers. Viscosity, the resistance to flow, is inversely proportional to shear rate. Thus, toward the wall of the vessel there is a larger difference in velocity (shear rate) but relatively low viscosity (i.e., less resistance). In contrast, toward the center of flow the difference in velocity is much smaller (v₆ – v₅)/d, hence a low shear rate, and viscosity is logarithmically higher (also see Fig. 2C).