system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do I get light headed and dizzy when I stand up too quickly?

So I was in my room and went to switch the light on, when I stood up I went all light headed and dizzy - u know - when someone says "woah I stood up too quickly, headrush!" Why reddit? Why? 

PS, I'm healthy, never had any brain issues, no intoxicants (although seems to happen to stoners lots more than non-smokers?) my resting pulse is just above 50, I don't smoke and there's no history of stroke or anything else! And no, it's naht a toomah. 

PPS, say yes to arnie, no to drugs.

(Edit - If your answer is something to do with rushing blood, why does *that* happen?!)
gpt
So you have numerous arteries leading into your skull and veins leading out of your skull. Their function is to supply your brain with fresh, oxygen rich blood and to drain it of the used, oxygen deplete blood. This whole system works because your heart's pumping your blood in a closed loop of arteries and veins throughout your body.

Now depending on certain physical positions you put yourself in, it can cause imbalances in the quantity of blood in different areas of your body. When you're off your feet, relaxing, perhaps reclined somewhat so your legs are elevated, there is going to be less blood in your legs than if you were standing. This has mostly to do with gravity not pulling blood downward towards your feet. Now if you stand up quickly, guess what happens? A bunch of blood that was previously not being pulled downward by gravity suddenly is being pulled down.

This drop in blood pressure slows the amount of blood that is going to your brain and you get a little light headed as a result. As soon as your brain starts feeling a little oxygen-starved though, it generally will tell your heart to get it's ass in gear and pump harder so as to bring more O2 to the brain quicker. That's why you often have a stronger pulse after a head rush, it's your body overcompensating because the brain think's it's starving.