system
Given the following post, respond as if you're commenting on Reddit:
human
Why is it that the blood rushes to someone's head when they're upside down but the blood doesn't rush to your feet when you're standing up?
gpt
The short answer is that it *does* pool in your legs when standing up. It works its way back up to hour heart because veins (as opposed to arteries) have one-way valves that prevent backflow and the contractions of our muscles squeeze it up as you use them.

Your head doesn't have the muscles that your legs do to get blood back to the heart; it relies on gravity to return blood to the heart.