system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:Why and how are bats able to hang upside down for long periods of time without all the blood rushing to their heads like it would for a human?
gpt
Due to gravity, blood is pulled towards your feet. Because humans stand up perfectly straight (a trait unique to us amongst mammals) your entire vascular system is designed to pump blood towards your head stronger than towards your feet; this counteracts gravity and results in healthy blood flow.

Because bats spend so much idle time upside down, their vascular system has evolved to do the opposite: pump blood away from the head to normalize the blood flow. During flight, the wings absorb so much blood that this effect is negligible.