Valves in veins are bicuspid, meaning they have two flap-like structures that regulate blood flow. These flaps are made of elastic tissue. When the valve is open, blood freely flows upward towards the heart and when the valves are closed, blood cannot flow back towards the legs. In your veins there are one-way valves that are placed about one inch apart that keep blood flowing in the right direction. When your leg muscles relax, the valves inside your veins close preventing the backward flow of blood back down the legs. https://www.usaveinclinics.com/blog/what-are-vein-valves/