Veins carry blood from the body parts back to the heart. Veins contain one-way valves to prevent the blood from flowing backwards. If the one-way valve becomes weak or damaged, some of the blood can leak back into the vein, collect there, and then become congested as the pressure builds. This congestion will cause the vein to abnormally enlarge. These enlarged veins can be seen on the surface of the skin as either varicose veins or spider veins.
Varicose veins are swollen and raised above the surface of the skin. Spider veins are similar to varicose veins, but they are smaller, are often red or blue in color, and are closer to the surface of the skin than varicose veins. They are also known as telangectasias. They can look like a tree branch or spider web with their short jagged lines. They can cover either a very small or very large area of skin. Spider veins are connected to larger vein systems through reticular veins. Spider veins aren’t necessary for circulation, rather they result when high pressure in veins from faulty valves stretch out the normally small and invisible surface skin veins forming abnormally distended and visible veins.
Because venous pressure is highest in the legs, spider veins are most commonly found on the lower extremities, but they can be found anywhere on the surface of the skin. Some estimates place more than half of the adult female population of the United States with spider veins.