eli5 how does our blood get back up when once it’s in our feet ?
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You’re right to think the heart can’t do it on its own. It already needs to force all those blood cells into single cell wide capillaries, it can’t possibly do that while maintaining the pressure to bring it back on the other side.
Which is why it doesn’t. Your veins, particularly the large ones in your legs, contain one-way valves which prevent blood from flowing backward. Whenever you flex the muscles in your legs by moving, this squeezes the veins, and the blood inside can only move in one direction due to the valves. In this way, the act of walking or running itself helps pump the blood back up to your heart.
This is one of many reasons why a sedentary lifestyle is so unhealthy. If you don’t move your legs all day, that blood is going to pool in your legs. Some of it will still flow due to random twitches, but it will be slow. This can cause blood clots, but also varicose veins as pressure builds up on those valves and they eventually fail. By moving, or working at a standing desk, you prevent this problem.
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