why does blood violently rush to your head when you’re upside down; yet, you could stand all day long.

Question

You can stand for long periods of time without issues, but can only be upside down for a short period of time before you get major head rush.

In: Physics

Answer 1

The human body has adaptations to help keep blood flowing to the brain despite gravity trying to pull the blood away and it also has adaptations to prevent blood from pooling in our legs coz of gravity causing us to have elephant legs, so if we turn up side down all of a sudden, all the blood rushes to our head because the body isn’t adapted to this relative gravity change. The reason why this is bad is because the blood vessels can only take so much fluid at once and so this fluid starts leaking into the tissues around the vessels because gravity prevents it from going anywhere else and in this case it leaks into the skull cavity and this extra fluid compresses the brain which is why you feel queezy and dizzy if you hang upside down because the brain is not happy.

If you’re wondering what these adaptations are, it’s the bodies ability to regulate blood vessel constriction in different ways all over the body to ensure each part of the body gets enough blood for the required energy usage it has at the time (blood vessels in muscles dilate during excercise to feed the energy demand), and normally it tries to maximise blood flow to the brain and minimise blood flow to the peripheries because gravity will do most of the work where as the blood that goes to the brain has to work against gravity. When you turn upside down, the body is still trying to maximise the blood flowing to the brain but it no longer needs this help since gravity brings it to the brain with ease, so you end up with too much blood flowing to the brain. There are a couple other Adaptions aswell such as The veins in the legs that return blood to the heart also have one way valves which prevent gravity from pulling the blood back down to the legs. Lastly the muscles in the legs act as a secondary pump to help pump the veins and return this leg blood back to the heart (this is why people on long flights get blood clots because the long period of time without standing up and moving means the blood in the leg pools for a long time without the leg muscles helping it pump, and the blood begins to clot like tomato sauce left on this tip of the sauce bottle)