How come, when you stand on your head, blood rushes to it, but nothing happens when you stand upright?

Answer 1

THe body was made to pump blood to your head when upright. THe heart is strong enough to push the blood up, fighting the force of gravity. If you are upside down, gravity is now working with your heart, pushing much more blood to your head and much less blood to your feet.

Answer 2

The "default" position for human beings is standing upright. The blood pressure in our bodies is normal in our heads in this position. Our legs and abdomen can be used to physically squeeze more blood to our brain if necessary, because our brain is more important for the body's survival than the legs. You'll notice that fighter pilots, when pulling very high G turns, will squeeze their legs and abdomen to force more blood to the brain so as not to black out. If we stood upright and all the blood rushed to our legs naturally, we'd be a species that was perpetually passed out.

Craig

Answer 3

Standing on ones head is a relatively unusual manoeuvre to which the body is somewhat untrained to react whilst standing up happens frequently (to most of us) and evolution has taught the brain how to react quite quickly.
However, as I know to my cost, as one gets older the body may not adjust blood pressure levels quickly and the result (if one does not wait foe a few seconds before setting off) is to pass out (faint) and often to do damage to oneself.
My worst experience of this kind (without alcohol!) was to fall in the centre lane of a three lane one-way system in Leeds with three lanes about to approach me on the change of traffic lights. I made the kerb on coming to and, the most pleasing thing was that three members of the passing public approached me offering help.
Would this have happened in London or New York?
It's great to be an honorary Tyke.

Answer 4

You have valves in the veins in your legs that prevent blood from flowing backwards to your feet.

Since you body is not designed to stand on your head, you have no comparable system in the veins in your neck.

Answer 5

Gravity

Answer 6

Your circulatory system isnt designed to work upside down, so the blood pools in the veins.

Answer 7

the human body is designed to operate in an up right or flat positon. our muscles and other organs work together. when we are turned up side down gravity takes an effect and your heart thinks its easier to pump blood to your brain.

Answer 8

humans are built to be upright, its how our heart works and our organs function, sloths, which spend almost their entire life upside down actually have their organs inverted so they can live that way.

Answer 9

yes it does- if you have low blood pressure like mine, you see stars if you stand up quickly! other than that, the other answers explain it well!

Answer 10

Because your head us above your heart. When you reverse it, it's like a backwards flush, it just doesn't work.