The function of veins is to return blood from the body back to the heart and lungs. Veins move blood passively, against gravity, by utilizing one-way valves with the surrounding muscles acting as pumps.
When a muscle is active and contracting, pressure within the vein is increased and blood moves forward.
When the pressure increase is stopped, blood is prevented from flowing backward by the one-way valves inside the veins.
If the pressure within veins becomes too great, the vein stretches out and the valve cusps may become too far apart to function. This is referred to as valvular incompetence.
In this instance the blood in the vein may become stagnant or flow in the wrong direction. This is referred to as venous reflux. Venous reflux causes veins to stretch and become visible; these are referred to as varicose veins. Over time, people may develop a heavy, aching sensation around these veins.
