When one of the leg veins, which carry blood from your feet back to your heart is damaged, blood flow back to the heart is reduced. As a result, you may develop vein problems in your legs. Leg vein problems begin when the vein valves that prevent blood from flowing in the wrong direction become weak or malfunction. This causes blood to pool in the deep veins of the leg. Over time, this pooling of blood will create pressure inside the vein walls and result in enlargement and sometimes bulging of the vein. These enlarged veins are the cause of spider veins (telangiectasia) and varicose veins. Varicose veins, a condition closely related to vein insufficiency, occur when veins near the surface of the skin are damaged.