Veins return oxygen-poor blood to the heart. Normal leg veins have one-way valves in them which help maintain blood flow against the force of gravity. The valves can become non-functional for a variety of reasons, to include genetics, hormonal changes, pregnancy, prolonged standing, and a history of blood clots. When the valves are not working correctly, gravity acts on the blood in the venous system, causing the veins to become swollen. Veins closer to the skin may seem to "pop out" of the leg and are visible as twisting, bluish cords, known as varicose veins. The underlying valve problem can also contribute to symptoms of leg tiredness, aching, pain, and swelling after standing for long periods. Untreated valve problems can progress to skin breakdown and the formation of an ulcer, or sore, located close to the ankle. The spectrum of treatable vein problems ranges from spider veins to symptoms and physical findings of venous insufficiency, including varicose veins and venous stasis ulcers.