Veins are designed to direct blood flow back to the heart after it has been pumped into the body through the arteries. Inside the veins are one-way valves, spaced two to three inches apart along the length of the entire vein, which are meant to keep the blood flowing in only one direction — back towards the heart. In the legs, the veins must transmit blood against the force of gravity upwards to the heart.
About 90 percent of the blood that flows through your legs is carried by deep veins, which are surrounded by muscle. They do not stretch easily because they receive strong support from the surrounding muscles. The veins under your skin outside the muscle compartment are called superficial veins. They do not have the support of muscles around them to prevent them from stretching and dilating. These veins have a tendency to dilate and develop faulty valves, resulting in gravity pulling blood backwards into the veins of the legs. This is known as venous reflux or venous insufficiency and eventually leads to leg symptoms such as pain and heaviness, varicose veins, skin changes, swelling and in some patients, venous ulcers.