system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why a person can’t survive upside down (head down - legs up) for a long time?
gpt
Our hearts have evolved to pump blood with a specific amount of pressure, utilizing gravity to assist us. We've always had gravity to help, and many of our bodily functions are dependent on it. 

Our hearts are not powerful enough to continually pump our blood through our veins when the force of gravity is acting against us.

This isn't a big deal in microgravity (like the ISS or space in general), because the force of gravity isn't acting against us in any way, so we're able to pump blood relatively normally.