Normally, veins should not be visible on the skin. When varicose veins start to appear, this reflects a problem in the venous circulation, a condition referred to as Superficial Venous Insufficiency. Under the influence of many factors, the blood flow becomes slower in the leg veins, and their pressure increases. These veins grow larger and become visible as varicose veins. Spider veins may be only of cosmetic concern but dilated varicose veins may cause serious complications.