The Fåhraeus–Lindqvist effect describes how the viscosity of a fluid, in this case blood, changes with the diameter of the tube it travels through. In particular there is a 'decrease in viscosity as the tube's diameter decreases' (although only with a tube diameter of between 10 and 300 micrometers). This is because erythrocytes move over to the centre of the vessel, leaving only plasma near the wall of the vessel. History The effect was first documented by a German group in 1930. Shortly after, in 1931, it was reported independently by the Swedish scientists Robin Fåhræus and Torsten Lindqvist, after whom the effect is commonly named. Robert (Robin) Sanno Fåhræus was a Swedish pathologist and hematologist, born on October 15, 1888, in Stockholm. He died on September 18, 1968, in Uppsala, Sweden. Johan Torsten Lindqvist was a Swedish physician, who was born in 1906 and died in 2007. Fåhræus and Lindqvist published their article in the American Journal of Physiology in 1931 describing the effect. Their study represented an important advance in the understanding of hemodynamics which had widespread implications for the study of human physiology. They forced blood through fine glass capillary tubes connecting two reservoirs. Capillary diameters were less than 250 μm, and experiments were conducted at sufficiently high shear rates (≥100 1/s) so that a similar flow in a large tube would be effectively Newtonian. After correcting for entrance effects, they presented their data in terms of an effective viscosity, derived from fitting measured pressure drop and volume flow rate to Hagen–Poiseuille equation for a tube of radius R where: is the volumetric flow rate is the pressure drop across the capillary is the length of capillary is the effective viscosity is the radius is the mathematical constant Although the Hagen–Poiseuille equation is only valid for a Newtonian fluid, fitting experimental data to this equation provides a convenient method of characterizing flow resistance by a single number, namely . In general, will depend on the fluid being tested, the capillary diameter, and the flow rate (or pressure drop). However, for a given fluid and a fixed pressure drop, data can be compared between capillaries of differing diameter. Fahraeus and Lindqvist noticed two unusual features of their data. First, decreased with decreasing capillary radius, R. This decrease was most pronounced for capillary diameters < 0.5mm. Second, the tube hematocrit (i.e., the average hematocrit in the capillary) was always