Page 9, Turbulence & Rheology, Dr. D. Penney

Fahraeus-Lindqvist Effect (continued..)

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 smaller 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. This is because erythrocytes being relatively large compared to the openings into small arterioles, have difficulty gaining entrance. Regardless of the precise mechanism, a relative decrease in erythrocyte number causes a decrease in the viscosity.

Thus, the Fahraeus effect can largely explain the Fahraeus-Lindqvist effect. Some other factors may also play a role. For example, the smaller the vessel the larger the relative volume of the slippage layer since the width of the slippage layer remains constant (~4 um). The high shear rate in the arterioles, and the increased erythrocyte deformation in small vessels, may also be important.

As a consequence of these mechanisms, the relative erythrocyte to plasma distribution varies according to vessel caliber. In a 1.1 mm diameter vessel the hematocrit may be 40.5% and the red cells and plasma move at the same velocity. In contrast, in a vessel with a 50 um diameter, the hematocrit may be 28% and the velocity of the red cells may be 175% that of the plasma. This strikingly illustrates the Fahraeus effect.

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