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
The ~~arteries~~ veins in your legs have sort of one-way valves which prevent blood from flowing back down.
In general, your body and circulatory system are just adapted to you walking upright (and having to fight gravity to get blood into your head) so you don’t feel ill effects from doing it all day long.
Your heart is not the only part of your body that helps pump your blood. Other parts of your body, like your feet, also contribute to ensuring blood is pumped.
When you are right-side up, this means you have support to pump blood UP from the feet, and your heart is in the upper portion of your body to get blood up to the head.
When you are upside-down, all the pumps are in the wrong places. Gravity pulls blood to your head, but you don’t have any muscle groups that pump it back out again.
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)
Your body was designed to walk upright, this is one of our unique ape characteristics
Some animals and bugs can hang upside down – because they are “built” for it
Some animals can live on the bottom of the ocean because they’re “built” for it
Same sort of thing. We were built to be upright.
Your body is eveolved for you being upright at a given amount of gravity, your blood vessels your heart are shaped in a way that facilitates a certain way of circulation.
That circlation includes fighting gravity in your arms and legs.
If you are upside down your body don’t have to fight gravity, rather it helps flow, so the blood rushes.
Like if you have a backpack for a long time and you take it off and feel lighter.
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This is similar to you raising your arm above your head for a long time. your body is not prepared to pump blood that hight so the blood rushes out.
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