| MadSci Network: NeuroScience |
That 'headrush' sensation comes from a decrease in blood being pumped to the brain (via gravity). When you stand blood rushes into your lower extremities. More blood in your legs means less going to your head, and so you probably feel a little dizzy or begin to see 'stars.' Usually your body adjusts to the change in a few seconds - vasculature in your legs is constricted and your heart beats a little faster to increase the output of oxygenated blood.
In some cases people do not adjust to the change - they are said to have orthostatic hypotension which simply means 'can't pump sufficient blood to the brain when standing.' Dehydration is the most common cause - less fluid to go around in general, particularly when standing. If severe the brain becomes so affected that the person faints, whereupon one's brain is once again on level with one's feet and thus able to receive sufficient oxygen.
-P.R.