Well, as it turns out, unlike the arteries, veins actually contain a system of valves. They have a system of one way of valves that prevents the backflow of blood inside the vein. So, to see what we mean, let's take a look at the following diagram. 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?