Veins transport blood from all other organs to the heart. As the heart is located in the upper region of the body, most of the blood carried by veins is from the lower organs, upwards, i.e. against the normal gravitational force. Therefore, they have valves to prevent back flow of blood due to gravity.

The arteries carry oxygenated blood from the heart to all other organs, i.e. from the upper part of the body, to the lower organs. In this, they are helped by the gravitational force and by the pressure difference between the two organs. The blood in arteries flows at a very high pressure. Thus, the arteries are aided by a number of factors that would carry blood in one direction only. Thus, they don’t require valves.