Veins are the blood vessels through which blood flows back to the heart. Blood that flows into the legs must circulate and then return to the heart. Because the blood needs to flow uphill and against gravity, the veins in the lower part of the body must work hard to move the blood back to the heart. Valves in the veins normally allow the blood to flow toward the heart, and prevent the back flow of blood. If these valves weaken and cannot perform their job, blood may pool in the legs, causing veins to bulge and dilate. When blood pools in the larger surface veins, the result can be the blue, bulging type of varicose vein. When this happens to the tiny veins just below the skin surface, the result can be spider veins.