Veins contain valves, which are integral to the circulatory system. They direct blood flow back towards the heart, supporting efficient circulation, especially against gravity. This design maintains a steady, unidirectional flow.
The Purpose of Venous Valves
Venous valves are essential because blood pressure in veins is significantly lower than in arteries. Without them, low pressure and gravity would cause blood to pool and flow backward, especially in the lower extremities. Their primary function is to prevent this backflow, ensuring deoxygenated blood consistently moves towards the heart and lungs.
These valves consist of two cup-shaped leaflets, formed from an inward projection of the vein’s inner lining, the tunica intima. These flexible leaflets open to allow blood to pass when propelled towards the heart. If blood attempts to move backward, the leaflets fill and snap shut, blocking flow.
Venous valves are assisted by surrounding skeletal muscles, particularly in the limbs. When these muscles contract, they compress veins, pushing blood forward. The valves’ one-way action ensures this muscular compression, known as the “muscle pump,” efficiently propels blood towards the heart, preventing backflow when muscles relax.
Where Venous Valves Are Located
Venous valves are most prevalent in the veins of the limbs, particularly the legs and arms. These regions are most susceptible to gravity, requiring mechanisms to facilitate blood return to the heart. Valves are found in both superficial veins, closer to the skin, and deep veins, situated within muscles.
While common in the limbs, valves are absent or scarce in major veins closer to the heart, such as the superior and inferior vena cava, common iliac veins, and most veins in the head and neck. In these areas, proximity to the heart, changes in thoracic pressure during breathing, and gravity assist blood flow. Valves are distributed to counteract gravitational pull where it poses the greatest challenge.
What Happens When Venous Valves Fail
When venous valves are damaged or weakened, they can no longer close properly, leading to venous insufficiency or reflux. This allows blood to flow backward and pool, increasing pressure within affected vessels. The pooling and increased pressure can distend veins, making them visibly enlarged and twisted, known as varicose veins.
Chronic venous insufficiency (CVI) can develop from persistent valve failure. This condition leads to symptoms beyond varicose veins, such as aching, cramping, or heaviness in the legs. Swelling (edema) in the ankles and lower legs is also common, as fluid leaks from overloaded veins into surrounding tissues.
Over time, chronic increases in venous pressure can cause skin changes. This includes discoloration, such as reddish-brown patches, and the skin may become thickened, hardened, or scaly. In severe cases, poor circulation and tissue damage can lead to venous ulcers, particularly near the ankles, which are slow to heal. Untreated venous insufficiency also carries a risk of complications, including skin infections and blood clots.