So, this is a simplified diagram of the vein. So, we basically have our blood that is moving against the force of gravity. So the question is, what exactly prevents the blood from actually stopping and going back down? 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.