text: Robert (Robin) Sanno Fåhræus, a Swedish pathologist, and hematologist, and Johan Torsten Lindqvist, a Swedish physician, observed that when blood flows through vessels smaller than about 1.5  mm in diameter, the apparent viscosity of the fluid decreases. The viscosity of blood decreases as the percent of the diameter of a vessel occupied by the cell-free layer increases. However, when the diameter of the tube approaches the diameter of the erythrocyte, the viscosity increases dramatically. For blood flow through tubes less than approximately 1  mm in diameter, the viscosity is not constant with respect to the tube diameter. Therefore, blood behaves as a non-Newtonian fluid in such blood vessels.