Veins return blood from your body to your heart and lungs so oxygenated blood can be re-circulated. The superficial veins under the skin are connected to the deep veins within the body by multiple bifurcating veins. In order to return blood to your heart, the veins in your legs must work against gravity. This is accomplished by toned elastic vein walls, muscle contractions that act as pumps in your lower legs, and tiny one-way valves scattered throughout the venous system. The valves open as blood is forced upward with each contraction of your leg muscles and then close to stop blood from flowing backward between leg muscle contractions. Varicose veins are the result of venous insufficiency or incompetence.

Superficial venous incompetence is the most common kind of venous disease. It occurs when the valves inside the veins malfunction or become damaged. This can occur due to injury, inflammation such as phlebitis, or due do congenitally weak vein walls. Any of the factors can result in increased pressure. The added pressure in the veins causes them to stretch, bulge and become varicose.

Medical studies have found that the composition of the walls of varicose veins is different from that of normal vein walls, which suggests that something inherently different from normal veins causes them to be particularly vulnerable to varicosity.

Exposure to sun or extreme cold and certain skin conditions like rosacea (facial redness) can cause spider veins to appear on the face. Spider veins on the legs are often of genetic or hereditary origin and directly related to increased pressure in the feeder veins.