their are valves in your vessels that stop it flowing the wrong way :)
because you heart uses different tubes. like how we dont get water in and water out of the house mixed up
through the veins and arteries? There are valves that control the direction of blood flow, it can only go one way
Because blood flows in one way and out the other :) We watched a video on it and learned a song about it. Jeesh it was horrible.