If the valves inside your veins become damaged, the valves may not close completely. This allows blood to flow in both directions. When your muscles relax, the valves inside the damaged vein(s) will not be able to hold the blood. This can cause pooling of blood or swelling in the veins.
The veins bulge and look like ropes under your skin. The blood begins to move more slowly through the veins. It may stick to the sides of the vessel walls and blood clots can form.