Typically, 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. Weakened valves, also called incompetent valves, within the veins might cause varicose veins. The weakened valves let blood pool in the veins instead of traveling to the heart. When blood pools in the veins, the veins become larger, making them show under the skin.
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