Poiseille's-Hagen law is related to

In the small blood vessels, essentially all the blood is near the wall, so extremely rapidly flowing central stream of blood doesn&;t exist. By integrating the velocities of all the concentric rings of flowing blood and multiplying them by the areas of the rings one can derive the formula known as poiseuille&;s law.In the equation, the rate of blood flow is directly propoional to 4th power of the radius of vessel.Ref: Guyton and Hall, Textbook of medical physiology, 2nd South Asia Edition, page no. 233