system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do bats’ circulatory systems tolerate being upside down for such long periods?

As far as I know, other mammals (I think they’re mammals) can’t tolerate this and the pooling of blood in the brain causes issues.
gpt
The quick answer is that Bats don't weigh enough for gravity to affect it's blood flow.
This includes the largest species, the black flying fox - it only weighs 1.1Kg.

As a human, we have litres of blood to move around so we have quite large arteries to help with that - so when we hang upside down there is alot of blood for gravity to affect, and an effective highway for blood to migrate towards our heads.

As a bat, they don't have nearly as much blood and their arteries are considerably smaller, so when they hang upside down they aren't affected in the same way.