Page 7, Turbulence & Rheology, Dr. D. Penney

Rheology (continued..)

Blood flow is not proportional to perfusion pressure. For example, viscosity is seen to drop sharply as shear rate increases (Figure 6.02), whether at a hematocrit of 60 or of 40. At higher shear rates, relative viscosity is fairly constant. In contrast, the relative viscosity of plasma and saline is absolutely constant regardless of perfusion pressure or shear rate.

The same data can be plotted another way, as flow rate versus perfusion pressure gradient (Figure 6.03). Plasma and saline again give straight lines, since they are Newtonian. Blood of different hematocrits display segments of non-linearity followed by linear behavior as flow becomes Newtonian.

Viscosity is equal to shear stress divided by shear rate. Shear rate is a measure of the relative movement of coincident concentric imaginary layers of fluid. Shear rate is the velocity of the cylinders next to one another, while shear stress is the horizontal drag between the concentric cylinders. [SEE MISCONCEPTION 21]

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