Remember, arteries bring blood into the legs (what we think of as blood supply) and veins return the blood to the heart. When you sit or stand, the blood in the veins is going uphill against gravity. One way passive flap-valves spaced every 2 to 4 inches in the veins assist the blood flow toward the heart. If the valves fail to function correctly, the blood pools down the vein causing congestion in the leg veins. This is known as venous reflux. It is the basis for the development of varicose veins, and can lead to symptoms such as pain, swelling, swollen limbs, leg heaviness and fatigue, skin changes and skin ulcers, and varicose veins.