system
Given the following post, respond as if you're commenting on Reddit:
human
What is actually happening when one "stands up too fast" and their vision goes dark temporarily?

I've heard it's something to do with blood rushing to one's head, and if this is the case, how does this cause the effect?
gpt
When you stand up fast, some blood pools in your legs and ankles, usually the body deals with this quickly so you don't notice it. This results in lower blood pressure to the brain. Dizzyness/fainting/dark vision, is usually a sign of the brain not getting enough oxygen. Sometimes if you stand up too fast you might even faint.