= Solution
The <Fahraeus--Lindqvist effect> is the decrease of blood's apparent or effective viscosity as a microvessel narrows through much of the physiological small-vessel range. Deformable red blood cells migrate away from the wall and concentrate near the centre, creating a <cell-free layer> of relatively low-viscosity plasma beside the vessel wall. Because the largest shear occurs near the wall, replacing cell-rich blood there by plasma reduces hydraulic resistance particularly effectively. At diameters comparable with a red blood cell, confinement eventually invalidates this decreasing trend.
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