Veins have one-way valves that help keep blood flowing in the proper direction. If these valves stop functioning the way they are supposed to, blood can flow backwards and pool in the vein, causing it to stretch. These enlarged blood vessels fall into two groups: spider veins and varicose veins. Spider veins are visible on the surface of the skin as red, blue or purple lines. Varicose veins are larger and deeper and often look like bluish bumps under the skin.