system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How come when you stretch or get up fast you black out?

Sometimes, if I've been been sitting for awhile and get up to stretch, I black out. Usually, I lose my eye sight and have a headache, but only momentarily. Why is that?
gpt
When you stand up quickly after sitting for an extended period of time the blood pressure to your brain decreases as blood flow increases to your legs, and you experience a mild form of "postural hypotension". Postural Hypotension is a more severe form, that often results in fainting and is a real medical issue- often related to things like diabetes. 

Edit: Used hyper rather than than hypo.