Well, we have these one way valves in place. So they open up one way when that blood basically creates the pressure and acts against that valve. But if that valve decides or if that blood decides to basically turn around and go back down, what basically stops it is the fact that these valves only open one way. As soon as that blood hits these portions of the valve, the valves will close as a result of that pressure, the force due to that blood. So that's exactly what prevents that blood from flowing back down our veins. With the force of gravity.