system
Given the following post, respond as if you're commenting on Reddit:
human
What causes head-rush when we stand up too fast? What do we "see" when this occurs?

As the title asks. When you stand up too fast from a sitting/laying down position and your vision goes dark and you start to see "images". What causes this?
gpt
When you are sitting down you are a bit compressed and when you stand up quickly, your blood suddenly has to fill the parts that didn't have so much blood due to the compression. This causes a drop in blood pressure resulting in the brain not getting enough blood, so it starts shutting down, which you feel as a head rush.