The paper presents the two-phase model to describe blood flow in large and in small blood vessels. Based on this model the study give 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.

Key words: hematocrit, Poiseuille flow, mathematical model, two-phase flow, relative viscosity.