The body develops a pressure difference, which enhances fluid flow by adjusting the radius of blood vessels. The pressure difference is basically what might determine the flow rate of a liquid through a pipe. It is directly proportional to the degree of high and low pressure on both sides of the pipe, and it is inversely proportional to the resistance of the pipe.
The blood vessel radius is responsible for the flow of blood. The pressure difference depends on it also. The flow of blood happens by contracting or relaxing the smooth muscle within the walls of the blood vessels. In other words, vasoconstriction or vasodilation. The blood vessel radius changes when the blood vessel radius increases as a person grow older and become an adult. The only time the length of the blood vessel will change is when a person adds or lose weight.
The pressure is higher when the heart pumps blood as compared to when relaxed. It pumps blood at a slower rate meaning less pressure exerted.
Blood that comes into direct contact with the walls of the vessels flows slowly because of the friction caused by the blood and the lining of the vessel. If it is the center, there is free movement because there is no contact or friction with the vessels. A phenomenon referred to as laminar flow.