Objective An analysis on the relation between the arterial branching angles and branch radii has been performed in detail. Methods Basic biomechanical principles are used in the analysis. Results The branching angle θ depends on the ratio of r/r 0:r is the radius of daughter blood vessel and r 0 is the radius of parent blood vessel. Their relation is cosθ=r/r 0. The average velocity of blood flow in any segment is proportional to r, and υ=υ 0 cos θ, where υ is the average velocity of blood flow in the daughter vessel and υ 0 is that in the parent. The smaller the daughter vessel radius, the lower the hematocrit in it, thus explaining the plasma skimming effect.Conclusion The cardiovascular system is not only a blood transport organ, but also a distribution system of blood flow and red blood cells