FahraeusLindqvist

The Fahraeus-Lindqvist effect is a phenomenon in microcirculatory blood flow in which the apparent viscosity of blood decreases as the diameter of the blood vessel becomes smaller, particularly in vessels ranging from about several micrometers up to a few hundred micrometers. It was described by Robin Fahraeus and Per Lindqvist in the 1930s through experiments using narrow glass capillaries. The effect is expressed as a reduction in the apparent blood viscosity relative to the viscosity of the blood in larger vessels or in bulk.

In small-diameter tubes, the blood exhibits a lower apparent viscosity than would be predicted by bulk rheology. This is accompanied by a decrease in the hematocrit within the vessel, known as the tube hematocrit, compared with the systemic hematocrit. A thin cell-free plasma layer forms near the vessel wall, while most red blood cells concentrate in a central core. This distribution reduces wall shear stress and overall flow resistance in many capillary segments.

The mechanism involves axial migration of red blood cells toward the center of the flow, driven by hydrodynamic interactions and shear-induced diffusion. Red blood cells deform and align with the flow in narrow channels, creating a marginated plasma layer at the periphery. The combination of cell-free plasma near the wall and a concentrated core lowers the energy required to sustain flow in small vessels.

The effect has significance for models of microvascular flow, influencing predictions of tissue perfusion and hematocrit distribution in capillaries. It depends on factors such as hematocrit, plasma protein content, and shear rate, and can be altered in disease states that affect red blood cell deformability or plasma viscosity.