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. So we see that even though the veins have the same exact three layers as our arteries, the veins have a much thinner tunica media. And they also have this system of one wave valves in place to prevent the movement of our blood back down our vein as a result of the force of gravity. Now, what about the third type of blood vessel, our capillaries?