Vein valves and the direction of blood flow can be seen on ultrasound. Ultrasound is an important tool in the diagnosis of venous reflux disease.

In the circulatory system, the veins carry de-oxygenated blood back to the heart. The leg veins carry blood toward the heart, against gravity. Therefore, the leg veins have one-way valves the prevent backflow of blood. When the valves do not function properly, they allow blood to flow backward, causing the pooling of blood. This is referred to as venous reflux or venous insufficiency. Eventually, the backflow of the blood will cause varicose veins to develop and symptoms related to the increased pressure in the leg veins such as leg heaviness, aching, swelling, restless legs, night cramps, throbbing, and pain.