system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What is that dizzy feeling you get when you suddenly stand up from where you were laying down?
gpt
That's the blood pressure in your head dropping, since gravity is suddenly pulling away from your head, down along your body, instead of evenly along your body's side. Specifically, lower blood pressure means less oxygen to the brain, which restricts its activity.

It's not particularly dangerous or harmful, but if you have low blood pressure, avoid standing up suddenly, as the drop in pressure may cause you to pass out.