Overview of the venous system

[Mary Mallozziberg]

Veins return blood to the heart from all the organs of the body. The large veins parallel the large arteries and often share the same name, but the pathways of the venous system are more difficult to trace than those of the arteries. Many unnamed small veins form irregular networks and connect with the large veins.
Many veins, particularly those in the arms and legs, have one-way valves. Each valve consists of two flaps (cusps or leaflets) with edges that meet. Blood, as it moves toward the heart, pushes the cusps open like a pair of one-way swinging doors. If gravity or muscle contractions try to pull the blood backward or if blood begins to back up in a vein, the cusps are pushed closed, preventing backward flow. Thus, valves help the return of blood to the heart—by opening when the blood flows toward the heart and closing when blood might flow backward because of gravity.
The body has

Superficial veins, located in the fatty layer under the skin
Deep veins, located in the muscles and along the bones

Short veins, called connecting veins, link the superficial and deep veins.
The deep veins play a significant role in propelling blood toward the heart. The one-way valves in deep veins prevent blood from flowing backward, and the muscles surrounding the deep veins compress them, helping force the blood toward the heart, just as squeezing a toothpaste tube ejects toothpaste. The powerful calf muscles are particularly important, forcefully compressing the deep veins in the legs with every step. The deep veins carry 90% or more of the blood from the legs toward the heart.
Superficial veins have the same type of valves as deep veins, but they are not surrounded by muscle. Thus, blood in the superficial veins is not forced toward the heart by the squeezing action of muscles, and it flows more slowly than blood in the deep veins. Much of the blood that flows through the superficial veins is diverted into the deep veins through the many connecting veins between the deep and superficial veins. Valves in the connecting veins allow blood to flow from the superficial veins into the deep veins but not vice versa.