eli5 why do people get dizzy when thay stand up to fast?
It happens more often when you have low blood pressure. Since BP lowers when you don’t eat, it gets worse.
I have a low BP and hence experience it quite often. It also happens to me when I sit down to relax after some hectic workout and then stand up.
It’s called [orthostatic hypotension](https://en.wikipedia.org/wiki/Orthostatic_hypotension). Wikipedia explains it as “predominantly by delayed (or absent) constriction of the lower body blood vessels, which is normally required to maintain adequate blood pressure when changing the position to standing. As a result, blood pools in the blood vessels of the legs for a longer period, and less is returned to the heart, thereby leading to a reduced cardiac output and inadequate blood flow to the brain.”
Basically, you stan up too fast and the blood vessels which would normally constrict and compensate for the change in blood pressure don’t quite work fast enough. You stand up, or sit up, and experience a short bout of dizziness. It’s normally harmless and almost everyone experiences it at some point.