Vein reflux disease, also known as venous insufficiency, is a medical condition affecting the circulation of blood in the legs. Arteries are blood vessels that carry oxygen-rich blood from the heart down to the legs. The "used" blood then returns up the legs to the heart through separate blood vessels called veins. When standing, the blood in the veins will flow up the leg against gravity. Veins have valves inside them that act as one-way flaps to prevent blood from flowing backward. If the valves break down or become damaged, the blood will flow backward on itself. Essentially, the blood will then get "backed up" in the vein and all the branches draining into this vein will also get backed up. Over time, more and more of the blood then "pools" in the veins and its branches causing the veins to stretch and enlarge. The veins closest to the skin are the ones we see as varicose veins. Vein reflux disease is the most common cause of varicose veins. Vein reflux disease is a progressive medical condition and may cause your symptoms to worsen.