Blood is a thixotropic substance. 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 several things: They move away from the wall, actually a distance of about 4 um, 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. This may be why arterial cushions, little hillocks surmounted by a branch vessel, are found in some vessels (e.g. umbilical artery). Blood taken near the wall would contain few erythrocytes, thus have a low oxygen content.
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. The blood initially resists movement and some of the driving energy has to be put into the blood to break cohesive bonds and to organize the red cells as described. Afterward the blood moves more easily and behaves much like a Newtonian fluid (e.g. water).
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