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Overview of the Venous System

By

James D. Douketis

, MD, McMaster University

Medically Reviewed Sep 2022
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Veins return blood to the heart from all the organs of the body. Arteries carry blood with oxygen and nutrients away from the heart to the rest of the body. The large veins parallel the large arteries and often share the same name. In addition, 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 flowing toward the heart pushes the flaps 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 flaps are pushed closed, so the blood does not flow backward. 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.

The body has

  • Superficial veins, located in the fatty layer under the skin

  • Deep veins, located in the muscles and along the bones

  • Connecting veins, which are short veins that link the superficial and deep veins

The deep veins play a significant role in pushing blood toward the heart. The one-way valves in deep veins prevent blood from flowing backward. The muscles surrounding the deep veins squeeze the veins, 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.

Deep Veins of the Legs

Deep Veins of the Leg

One-Way Valves in the Veins

One-way valves consist of two flaps (cusps or leaflets) with edges that meet. These valves help veins return blood to the heart. As blood moves toward the heart, it pushes the flaps open like a pair of one-way swinging doors (shown on the left). If gravity or muscle contractions momentarily pull the blood backward or if blood begins to back up in a vein, the flaps are immediately pushed closed, preventing backward flow (shown on the right).

One-Way Valves in the Veins

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. Therefore, 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.

Problems With the Veins

The main problems that affect the veins include

The veins in the legs are particularly at risk of blood clotting or swelling of the vein because when a person is standing, blood must flow upward from the leg veins, against gravity, to reach the heart.

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