Arteries carry blood from your heart out to your extremities (hands, feet, head, skin), delivering oxygen deep into the tissue. Veins then return the ‘de-oxygenated' blood (now bluish in color) back to your heart to be re-circulated. Nearly 75% of the body's blood is found in your lower limbs (legs).
To return this blood to the heart, your leg veins must work against gravity. Muscles in the leg squeeze the deep veins to help push blood forward. Small, one-way valves in the veins open to allow blood to flow upward, towards the heart, and then close to prevent it from flowing backwards. While deep veins are assisted in their efforts by muscles, a second type of leg vein, lying outside the muscle layer and closer to the skin (superficial veins), are not. The largest superficial vein is called the Great Saphenous Vein (GSV), which begins at the ankle and ends at the groin.
Varicose veins occur when the valves in these superficial veins malfunction. The vein walls can lose elasticity (due to age or hormones) causing them to stretch. When this occurs, the valve may be unable to close, allowing blood that should be moving towards the heart to flow backward (called venous reflux). Blood collects in your lower veins causing them to enlarge and become varicose. In this manner, faulty valves high on the leg may cause varicose veins lower down (e.g., mid or lower leg).