Varicose veins are swollen and enlarged veins that are close to the surface of the skin varicose veins develop when the small valves inside the veins stop working properly in a healthy vein blood flows smoothly to the heart the blood is prevented from flowing backwards by a series of tiny valves that open and close to let blood through if the valves weaken or are damaged the blood can flow backwards and can collect in the vein eventually causing it to be swollen and enlarged varicose