system
Given the following post, respond as if you're commenting on Reddit:
human
[EL5] Why do I almost pass out if I stand up and immediately stretch?

This happens to me, and my oldest son.  It's been happening to me since I was in my teens, and I understand it's not that uncommon.  If I just stand, it never happens.  If I just stretch, it doesn't happen.  What is it about standing and stretching that causes tunnel vision, light headedness, and (rarely) loss of conciousness?

I guess I know that it has to do with blood pressure, but why is our body so poor at regulating blood pressure when going from sitting to standing, and why does stretching exacerbate it so much.
gpt
Basically from gravity. Same thing happens if you stand up for a long period of time, your blood will collect in the lower portions of your body. When you sit, your body isn't worried about pumping a lot of blood, as you're essentially at rest. When you suddenly stand and stretch, your body goes into active mode, and with the blood at your extremities, your brain doesn't have as much oxygen as it would like, so for a short time you get effects similar to what would happen during asphyxiation.

It also ties into how physically fit you are, to someone extremely fit such as an athlete, they probably won't notice this as much, as their heart pumps harder than someoone who isn't quite as fit. For someone who is incredibly out of shape or just obese, this can cause them to pass out in extreme cases, as their heart simply cannot pump the amount of oxygen carrying blood needed to the brain, so it involuntarily shuts down to recover.