Varicose veins can develop through a combination of weakened vein walls and faulty valves. Under a variety of circumstances, normally toned, elastic vein walls can become weak and allow the recirculating blood that should be moving toward your heart to flow backward. One-way valves in these veins – which open and enable blood to flow through on its way upward, then close to stop blood from flowing backward – can also fail to function properly. This allows blood to pool and pressure to build up, which further weakens and subsequently damages the veins, causing them to become twisted, enlarged and painful. Up to 40% of women and 25% of men are affected by this condition – but fortunately, treatment is available.