Originally Posted by DrPete
The valves fail open, but you're forgetting that gravity is at play too. The reason that veins in the legs have one-way valves is because gravity is working on the blood too, and when standing your legs are far below the pump. So the pressure in the deep veins of the legs is relatively high. That's why patients who have symptoms generally only notice them when standing or walking for a while. If you elevate your legs, for instance, gravity is working for you so the blood will go back to the heart even if the valves are incompetent.
.
Well the pressure in your leg veins would be the same whether the valves are working or not - because the elevation gain back to your heart is the same. The valves help to prevent backflow in between pulses of pressure.
I'm seeing this from a hydraulic engineer viewpoint, not a medical one, but physics is physics.