To understand what causes varicose veins, you have to understand how your veins work. Unlike arteries, which contain muscle tissue to pump blood from the heart, your veins rely on a series of tiny, one-way valves to control the flow of blood back to the heart and the lungs. When the muscles surrounding the veins contract, these valves open to allow blood to flow towards the heart, and then close to prevent it from flowing backwards, back into the veins. One of the most common causes of varicose veins is chronic venous insufficiency, which is a condition in which these valves become diseased or damaged and don't close properly. As a result, the blood flows back into the diseased veins and collects there, causing them to become enlarged and discolored (the blue color comes from the deoxygenated blood pooling in the veins).