system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: what would happen to a person being turned upside down indefinitely? What is happening inside the body?
gpt
The human body normally works with gravity as much as possible, and has to develop methods to work against it. Our legs have a very powerful hookup to pull the blood back up from the bottom, for example.

However, our brain doesn't. Normally, blood will flow back out of the head with little effort due to gravity. If placed upside down, the blood would begin to pool in the head, as the vessels there are not expecting to work against gravity.

Eventually, this would cause the blood vessels to burst. Apparently, reverse hanging was a fairly common form of torturous execution in the past, and they would frequently right the victim every 4 - 6 hours to let the blood leave the head -- the idea being not to provide relief, but to extend the suffering.

Supposedly, it's lethal after 10 - 24 hours. It obviously depends on the level of physical fitness.