The venous system
Veins throughout your body return blood back to your heart. In the legs the veins are often carrying blood against the force of gravity. In order to do this efficiently, veins have one-way valves to prevent blood from flowing backwards. If the valves fail to close properly, blood can leak through the valves and pool in the veins.
Eventually this pooling causes stretching of the vein walls resulting in enlargement and bulging of the vein. These enlarged veins are called telangectasias (or spider veins) and varicose veins.
Spider veins are thin, red, purple or blue veins and are located just below the skin’s surface. Varicose veins are larger veins deeper under the skin that are abnormally enlarged. Varicose veins can eventually protrude from the skin surface, resulting in a rope-like appearance.