The leg contains two vein systems that provide circulation to the lower extremities. These are called the deep system and the superficial system.

The Deep Venous System

Veins in the deep system are in close proximity to the bone and are surrounded by the leg muscles.

The Superficial Venous System

The superficial veins are above the muscle, located in the fat tissue that lies beneath the skin. These are the veins that may sometimes be visible.

The two systems meet at two junctions: the groin and behind the knee. There is also a smaller system of veins where the two systems meet. These are called perforators.

When blood travels downward through the legs, gravity ensures that the blood flows smoothly. The body doesn't have this benefit on the way back up to the heart, so the leg has an alternative method of circulating blood. The combination of the deep and superficial systems, the leg muscles, and the various valves throughout the lower venous system form a pumping reflex secondary to the heart. The entire mechanism is often referred to as the second heart.

The Second Heart and Varicose & Spider Veins

The second heart is responsible for pushing deoxygenated blood back up to the heart and lungs, for recirculation throughout the body. The muscles in the lower leg and calf contract to pump the blood back up, while the valves open and close opposing the muscle contractions to ensure there is no backflow of blood back into the leg.

Failure of the Lower Circulatory System

When these valves become defective, the second heart system ceases to work properly. Blood backflows into the veins which causes the blood to pool. The pressure created from this overflow of blood in the veins causes the superficial veins to bulge. Depending on the severity of the problem, this leads to various vascular diseases, including varicose veins and spider veins.