So what allows that blood to move through our veins? Well, as it turns out, next to our veins are skeletal muscle. And we can actually voluntarily control our skeletal muscle. 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?