The paper presents the two-phase model to describe blood flow in large and in small blood vessels. Based on this model the study gives an explanation for long known features (effects) of blood flow in the vessels: dependence of the hematocrit (packed cell volume) on the diameter of the vessel, existence of cell-free plasma layer near the vessel wall, obtuse (as compared with the profile of Poiseuille flow) velocity profile of a blood; dependence of the blood viscosity on the diameter of the vessel. The researcher determines dependences of the blood rate and viscosity on the diameter of a blood vessel.