Section 3. Heart and Blood Vessel Disorders

Chapter 36. Venous Disorders

Introduction | Deep Vein Thrombosis | Superficial Thrombophlebitis | Varicose Veins | Arteriovenous Fistula

Introduction

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 return blood to the heart--by opening when the blood flows toward the heart and closing when it tries to flow backward.

The main problems that affect the veins include inflammation, clotting, and defects that lead to distention and varicose veins. The veins in the legs are particularly affected because when a person is standing, blood must flow upward from the leg veins, against gravity, to reach the heart.

The legs contain superficial veins, located in the fatty layer under the skin, and deep veins, located in the muscles. Short veins, called connecting veins, link the superficial and deep veins.

The deep veins play a major role in propelling blood upward. 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 upward, just as squeezing a toothpaste tube ejects toothpaste. The powerful calf muscles are particularly important, forcefully compressing the deep veins with every step. The deep veins carry 90% or more of the blood from the legs toward the heart.

See the figure One-Way Valves in the Veins.

Superficial veins play only a minor role in carrying blood to the heart. They 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 upward by the squeezing action of muscles, and it flows more slowly than blood in the deep veins. Much of the blood that flows up 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.