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Valves in veins are bicuspid, meaning they have two flap-like structures that regulate blood flow. These flaps are made of elastic tissue. The valves’ main job is to keep the blood moving in one direction – back up towards the heart. When the valve is open, blood freely flows upward towards the heart; when the valves are closed, blood cannot flow back towards the legs. The deep veins carry 90% or more of the blood from the legs toward the heart.
https://www.usaveinclinics.com/blog/what-are-vein-valves/
