Page 8, Turbulence & Rheology, Dr. D. Penney
When blood is allowed to flow through capillary tubes of decreasing caliber, a second non-Newtonian characteristic is observed. Below a critical vessel caliber, blood viscosity becomes dependent upon vessel radius. The critical radius is approximately 1 mm. Viscosity falls sharply down to a vessel caliber of approximately 12-15 um. This phenomenon is known as the Fahraeus-Lindqvist, or sigma effect (Figure 6.04). The precise explanation is very complicated, and is still not thoroughly understood. Probably the most important factor in explaining the Fahraeus-Lindqvist effect is the Fahraeus effect. Professor Fahraeus was lucky enough to get his name attached to two phenomena.
In the Fahraeus effect the dynamic hematocrit (determined by microcineangiography), the hematocrit of blood when it is actually moving as opposed to the bulk hematocrit which is measured after the blood is drawn and spun down, decreases below the bulk hematocrit in tubes down to a diameter about 15 um. For example, if blood were drawn from the body an hematocrit value of 45% may be obtained. This is a clinically normal value.
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