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.
The precise explanation is very complicated, and still not thoroughly understood. Probably the most important factor in explaining the Fahraeus-Lindqvist effect is the Fahraeus effect. In the phenomenon the dynamic hematocrit, 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, a clinically normal value. Dynamic hematocrit determined in arterioles and like vessels may have a value of only 25%. One reason for this discrepancy is that the erythrocytes pass through these tubes much faster than the plasma, because they are near the axis while most of the plasma is near the wall.
A second reason is that erythrocytes are excluded from the smaller vessels, in a process called plasma skimming or red blood cell screening. Thus, the flow behavior of blood is distinctly different from that of water.
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