You can stand for long periods of time without issues, but can only be upside down for a short period of time before you get major head rush.
In: Physics
The ~~arteries~~ veins in your legs have sort of one-way valves which prevent blood from flowing back down.
In general, your body and circulatory system are just adapted to you walking upright (and having to fight gravity to get blood into your head) so you don’t feel ill effects from doing it all day long.
Latest Answers