hen blood is being pumped to your feet, gravity helps take it on its way. To get blood to your head your body has evolved to pump harder.
But while there are muscles that help get the blood back from the legs, the brain doesn't have these same muscles. So it doesn't take long before blood begins to pool in places where it shouldn't, like the lungs and head.
"The major concern is about the control of the vascular system," says Professor Ashley Grossman, of Barts Medical School, part of Queen Mary, University of London.
A bat
Bats sleep upside down
"We have been designed over a million years to try and control blood pressure in terms of gravity from the top to the bottom. We can all cope in the short term but eventually when the blood starts pooling in the lungs and the head problems occur."
And these problems are bad. In the lungs, the risk is of a pulmonary oedema.
"Fluid will start to ease out of the blood vessels, you can't easily get oxygen, the lungs become rather stiff and you get breathless," says Professor Grossman.
"You might be able to breathe a bit faster but I doubt if that's going to work for more than a few hours before you get exhausted."
But as bad as this fluid on the lungs is, there is a more catastrophic danger for the upside down publicity seeker. The pooling of blood in the brain can cause death.