Our venous system

A widely branching system of blood vessels transports our blood around our bodies.

Distensibility

A very important feature of the vascular system is that blood vessels are distensible. That allows arteries to accommodate the pulsating blood stream from the heart and provide smooth blood flow throughout the arterial system. Veins are far more distensible than arteries, since their walls are thinner. Even slight increases in venous pressure cause increase in the blood content in venous system, about eight times more than in arteries. By this feature, veins provide a blood reservoir and can store large volumes of blood that can be used whenever additional blood flow is required elsewhere in the circulation.

Importance of the venous system

The venous vessel network is far more branched and complicated than the arterial one.

The venous system transports the deoxygenated blood from the body back to the heart and from there onwards to the lungs. Capillaries and venules, collect the used, deoxygenated blood from all over the body and pass it onto the veins for return transport to the heart.

Due to its build, the venous system has a function of blood storing pool and it contains up to 70% of the blood in the circulation. This reservoir function of veins makes them able to adjust the volume of blood returning to the heart, so that the needs of our body can be matched when changed.

Large veins, particularly those in the arms and legs, have one-way valves. Each valve consists of two flaps (cusps) with meeting edges. As the blood flows toward the heart, it pushes the cusps open like a swinging door. Once the blood has passed through them, these “doors” close by the blood that passed now pushing them backwards with its weight. Therefore, if the gravity pulls the blood back (particularly in legs’ veins) or the blood itself accumulates in a large amount and pushes back, the cusps are pushed closed by the blood, preventing backward flow. Consequently, 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.

Our venous system pumps approximately 7,000 litres of blood flow back to the heart every day.

Anatomy of the venous system

Veins are classified as:

• Deep veins, located deep inside our body, in the muscles and along the bones
• Superficial veins, located in the fatty layer under the skin
• Short veins, called connecting veins, link the superficial and deep veins.

Deep veins have a role to propel blood toward the heart. Their one-way valves prevent blood from flowing backward. These deep veins are laid inside the muscles,

so the surrounding muscles compress them when muscle is active (contracting, moving), thus forcing the blood toward the heart. For example, calf muscle is forcefully compressing the deep veins in the lower leg with every step we make and pushes the blood up the leg and toward the heart.

Superficial veins also have valves, but they are not surrounded by muscles. Subsequently, blood in the superficial veins is not forced by the squeezing action of muscles, and it flows more slowly than blood in the deep veins. Considerable volume of the blood is diverted from superficial veins into the deep veins, through the short veins that connect them.

The ability of veins to function depends on their distensibility, or compliance. Veins in legs are less compliant than those at or above the level of the heart. Veins in the legs are also thicker than those in the brain or in arms. The compliance of veins, decreases with age, and they become less elastic due to reduction in elastic fibres in their walls and an increase in collagen content.

Veins problems

The most common problems that affect the veins include:

Varicose Veins
A varicose vein is a vein in which blood has pooled, producing distended and swollen superficial vein that looks twisted and may become painful, itching or cause a sensation of tiredness. The most often are in legs and more likely in women than in men.

They may be a consequence of several causes, the most important one being a weak vein wall. This weakness of the wall may be inherited or acquired with age (loss of elastic fibers), obesity or repeated prolonged standing.

When a vein gets distended and elongated the cusps in one-way valves separate and can’t close properly. Under the influence of gravity, a section of the vein below such a deficient valve is subjected to the pressure of a larger volume of the blood. The vein enlarges as it becomes distended and surrounding tissue becomes swollen due to the increased hydrostatic pressure.

Spider veins
Spider veins are enlarged small venous capillaries, mostly in the skin. They can occur as a consequence of varicose veins or can be caused by hormonal factors (particularly in women during pregnancy).

Spider veins cause no pain or other issues. However, being visible on the skin, they might look unappealing.

Chronic Venous Insufficiency (CVI)

Varicose veins and failure of one-way valves can progress to chronic venous insufficiency, especially in obese individuals. Chronic venous insufficiency (CVI) is impaired venous return (blood flow from periphery toward the heart) causing discomfort, swelling, and skin changes. Extremities can become so inactive that their metabolic demands to be supplied with oxygen and nutrients and released from

wastes are barely met. In this situation, any trauma or pressure can cause cell death and necrosis (venous stasis ulcers).

Thrombosis (Thrombus formation in veins)
A thrombus is a blood clot attached to a blood vessel wall. It can be formed in arteries, but venous thrombi are more common.

Three factors promote venous thrombosis:

• venous stasis (e.g., immobility, age, congestive heart failure),
• damage of veins inner surface (e.g., trauma, intravenous medications), and
• conditions in which blood becomes prone to clotting (hypercoagulable states, e.g., inherited disorders, malignancy, pregnancy, use of oral contraceptives or hormone replacement therapy).

Blood clots (thrombi) can occur in the deep veins - deep vein thrombosis (DVT), or in the superficial veins - superficial venous thrombosis.

The superficial veins are usually also inflamed but without clotting (or thrombosis) - superficial thrombophlebitis.

Superficial venous thrombosis is inflammation and clotting in a superficial vein, usually in the arms or legs. The skin over the vein becomes red, swollen, and painful. It is usually treated by elevation, rest and pain killers.

Deep vein thrombosis occurs most often in the legs or pelvis but may also occasionally develop in the arms.