Weak or damaged valves can lead to varicose veins. Arteries carry blood from the heart to the rest of the body. Veins return blood from the rest of the body to the heart. To return blood to the heart, the veins in the legs must work against gravity. Muscle contractions in the lower legs act as pumps, and elastic vein walls help blood return to the heart. Tiny valves in the veins open as blood flows toward the heart, then close to stop blood from flowing backward. If these valves are weak or damaged, blood can flow backward and pool in the veins, causing the veins to stretch or twist.