Page 11, Turbulence & Rheology, Dr. D. Penney
Erythrocyte / Plasma Inhomogeneity:
Cell/plasma inhomogeneity in the circulation, for the reasons discussed above, is greater than might at first be imagined. A measure of central hematocrit is obtained by taking blood from any of the larger blood vessels in the body. It is the central hematocrit that is normally measured in the hospital and in a physician's office. A higher hematocrit yet is obtained if blood is taken from the spleen, since the spleen acts as a reservoir for red blood cells. A higher and lower hematocrit value may also be obtained in some veins where sludging occurs.
Sludging is produced by sluggish blood flow in veins or venous sinuses where separation of the erythrocytes and the plasma occurs under the influence of gravity. The same separation process is enormously speeded up in the centrifuge by taking advantage of slight differences in the relative densities of the components. A very low or high hematocrit is obtained depending upon where sampling is done in such a sludged vein. As discussed earlier, lower hematocrit blood predominates in the microcirculation due to the Fahraeus effect.
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