Page 16, CV Review, Essentials, Develop. Physiol., Dr. D. Penney

Misconception 19. Blood flows like water (i.e. constant viscosity regardless of velocity or tube dimensions).

When blood is not flowing or flowing very slowly in a blood vessel, the red blood cells move around in a random fashion. Some are near the center of the tube, while others are near the wall. They are not in any particular orientation. When the blood starts to move due to increased perfusion pressure, the red blood cells do a couple of things.

They move away from the wall, actually a distance of about 4 um away as predicted by an equation written by Albert Einstein. They move toward the axis of the tube, so they become oriented with respect to each other and the wall. They form a concentrated plug of erythrocytes moving near the tube axis. This leaves a relatively cell-free layer near the wall of the vessel. In so doing, the viscosity of the fluid near the wall where most of the friction is produced, is decreased. This phenomenon occurs in larger vessels such as the aorta, down to vessels the size of arterioles.

Energy is also put into breaking up Rouleaux formations, where erythrocytes stack one next to the other due to their sticky plasma membranes. This illustrates why when blood first begins flowing, the relationship of perfusion pressure or shear rate to flow is initially non-linear. This is one characteristic of a non-Newtonian fluid (Table 1.03).

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