system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why when upside-down does blood all go to our head, but when standing up blood flow is normal?
gpt
The normal state of our bodies involves having to pump blood "towards our head" against gravity to reach our brains. This is different from the idea that we pump blood "up". When you are turned upside down, your body is still doing the work of pumping the blood "towards your head" but now it is also having gravity pull the blood towards your head too instead of away from it.