Venous valves
Valvulae venosae
Venous valves are pairs of half-moon-shaped cusps formed from folds of the inner lining (tunica intima) of a vein. They let blood flow only toward the heart. They are most numerous in the veins of the limbs, where they work with the skeletal muscle pump.
Overview
Structure. Most venous valves have two cusps. Each cusp is a fold of intima with a thin core of connective tissue, covered by endothelium on both surfaces. Just behind each valve the vein wall bulges slightly to form a valve sinus. Blood flow is slowest in these pockets, which is why deep vein thrombi usually start there.
Distribution. Valves are most numerous in the lower limbs, increasing toward the foot. They are also present in the upper limb veins, the subclavian vein and near the end of the internal jugular vein. They are absent from the venae cavae, the hepatic portal system, the dural sinuses and cerebral veins, the pulmonary veins, most visceral veins and the smallest venules.
Function
- Prevent backflow of venous blood, especially when standing.
- Allow the skeletal muscle pump to propel blood toward the heart.
- Direct blood from superficial to deep veins through the perforating veins.
- Physiology
Muscle pump. When the calf muscles (soleus and gastrocnemius) contract, they squeeze the deep veins inside their tight fascial compartments and push blood upward. Valves stop it from falling back. This is why the calf is often called the "peripheral heart". When the muscles relax, the deep veins refill, partly from the superficial veins through the perforators.
Effect of walking. In a person standing still, venous pressure at the ankle is about 90 mmHg, equal to the weight of the column of blood up to the heart. During walking the muscle pump lowers it to around 20–30 mmHg. Breathing also helps venous return, because the pressure changes in the chest and abdomen draw blood toward the heart (the respiratory pump).
Clinical significance
Chronic venous insufficiency
Valves can fail because they are inherently weak (primary) or because they were destroyed by a previous thrombosis (post-thrombotic syndrome). Pressure in the leg veins then stays high even when walking (ambulatory venous hypertension). This causes swelling, varicose veins, haemosiderin pigmentation, eczema, lipodermatosclerosis and venous ulcers in the "gaiter" area above the medial malleolus. The CEAP system is used to classify its severity.
Fainting on parade
A soldier standing still for a long time has no muscle pump action. Blood pools in the legs, venous return and cardiac output fall, and the soldier may faint.
Compiled from standard anatomy and physiology references — see the Cardiovascular System sources. For education only; not medical advice.