In 2009, a Utah man named John Jones died after spending 28 hours stuck upside down in the Nutty Putty Cave. Rescue workers tried to work fast, but the walls of the passage were so narrow, they weren’t able to get him out before he died — most likely of asphyxiation.

Impact on Lungs

Turns out, your lungs evolved to sit atop of all the other organs for a reason. Delicate organs that they are, it doesn’t take them long to get squished by the bigger, heavier organs like the liver and intestines that usually sit below them.

This isn’t as much of a problem when you’re lying on an incline with your feet slightly elevated above your head, but when your head is directly underneath your feet, your lungs simply can’t absorb enough oxygen given the available space they have to work with.

But it’s not just the lungs you have to worry about during extended periods of reverse suspension. Our bodies are well set up to move blood around when we’re standing upright, and our blood vessels are customized to make sure blood doesn’t pool up in our feet.

But that system is a one-way street — our bodies didn’t evolve to keep blood from pooling in the brain. When this happens, all sort of things could go wrong, including ruptured blood vessels, which can lead to brain hemorrhage.

Impact on Heart

Medical professionals think heart failure is the cause of death in most upside-down fatalities, for much the same reason our brains can suffer: When you’re head down, your heart slows down its pumping and starts receiving more blood than it has the capacity to manage at one time. It begins to have a hard time maintaining blood pressure, and eventually loses its ability to move enough blood around to maintain all the body’s essential functions.