The veins in your legs and arms carry blood back to the heart and then on to the lungs to be oxygenated, i.e. carbon dioxide is removed and replaced with oxygen. Essentially the venous system is divided into two interconnected groups, i.e.
superficial and deep. The connecting or communicating veins are called perforators.
Unlike arteries, veins do not have the direct driving force of the heart to push blood around the body and must also work against the forces of gravity. Muscles in your arms and legs contract to squeeze blood through the veins. A series of small valves situated along the length of the vein helps the flow of blood back to the heart. These one-way valves prevent blood from falling backwards or regurgitating because of gravity or excessive pressure. A good analogy to the venous drainage system in your leg is a series of lochs in a river that helps move boats up a hill.
The heart pumps oxygen-rich blood through thick-walled arteries to all the tissues of the body. The veins return the blood to the heart. These thin-walled structures contain a series of unidirectional (one-way) valves that keep the blood flowing from the superficial veins located close to the skin’s surface to the deep veins found in the muscles of the arms and legs and towards the heart. Blood is propelled forward or antegrade by muscle contraction (squeezing), through the open valves which allows the blood to flow upwards to the heart. When the muscles relax, normal (competent) valves snap shut, preventing blood from flowing backward (retrograde).
There are three sets of veins in your legs. The superficial veins lie just under the skin and carry 10-15% of the blood back to the heart. Superficial veins connect via the communicating veins to the deep veins which lie within the muscles (responsible for pumping) and carry 85-90% of the blood back to the heart.
If the vein walls are weak and damaged, or if the valve is stretched or injured, the system becomes incompetent and blood is allowed to collect and flow in a retrograde (backward) fashion when the muscles relax. This creates unusually high pressure buildup in the veins, resulting in more stretching and twisting, increased swelling, more valve incompetence, sluggish blood flow, and potential blood clot formation. Over time, this phenomena contributes to a variety of disorders known as venous disease including varicose veins, deep venous thrombosis (DVT) or “blood clots in the deep veins”, and chronic venous insufficiency (chronically swollen or edematous legs).
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.
One-way valves consist of two flaps (cusps or leaflets) with edges that meet. These valves help veins return blood to the heart. Blood, as it moves toward the heart, pushes the cusps open like a pair of oneway swinging doors (shown on the left). If gravity momentarily pulls the blood backward or if blood begins to back up in a vein, the cusps are immediately pushed closed, preventing backward flow (shown on the right).
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.