Below is an instruction from Human. Write a response.
    ### Instruction:
    What causes Varicose Veins ?
    ### Response:
    Weak or damaged valves in the veins can cause varicose veins. After your arteries and capillaries deliver oxygen-rich blood to your body, your veins return the blood to your heart. The veins in your legs must work against gravity to do this.
                

    