Venous Valves

Venous valves are forced open by pressure from below the valve (i.e. during walking). How are they closed?

When the leg is in motion (walking), muscle contractions squeeze the veins, forcing blood past the open valves of the deep, superficial and perforating veins upward towards the heart. After the muscle relaxes, valves close to prevent backflow (reflux). Simply, the valves are forced open by pressure from below the valve (i.e. during walking) and are closed by pressure from above (i.e. standing or inspiration).