It is proposed a physiological and experimentally confirmed explanation of Fåhraeus-Lindqvist-effect in capillaries using the profile analyses of osmotic deformability of red blood cells. It was shown the dose-dependent change of the erythrocytes deformability in the stage of isotropic spheres after forming artificial water pores (nystatin) and occlusion (PbCl2) of available pores. The Sigma-effect reducing of hematocrit and viscosity in a shear flow of blood through the vessels of a small diameter was conditioned by the interchange of liquid phase between the erythrocyte and the plasma.