And that's exactly why when we move, when we run, when we swim, anytime we move our legs, that helps the movement of the blood inside our veins. That's one thing that assists the movement of our blood inside our vein. Now, what about the problem of that blood going back down the vein as a result of the force of gravity? 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.