The veins in our legs return blood to the heart as part of normal circulation. The machinery for this process is quite remarkable given the challenges of bringing blood up from the floor against gravity. To do this, veins have tiny valves that work like a ladder to keep blood moving toward your heart.
If the valves become dilated or damaged due to venous insufficiency, blood flow becomes impaired and the veins swell from the pooling of blood. Swollen veins visible on the surface develop as spider veins and varicose veins.