# Why can human body sustain relatively high (e.g. up to 30 atm) external pressure?

I found out from the Internet that for average people it's fine to sustain up to 30 atm external pressure, for example when diving. However, given a typical human palm size of 80 cm$$^2$$, 1 atm means about 80 kg of weight over the palm, which is the typical weight of a male adult. Then 30 atm over the palm means putting the palm on a hard surface and letting 30 male adults standing on one side of the palm (so that the surface will exert a roughly equal amount of pressure on the other side of the palm).

It's hard for me to imagine that the hand can withstand such a process. Is there any pitfall in my reasoning? How can I be convinced of the relatively high pressure tolerance of human body with this palm picture in consideration?

You can not pop a balloon by increasing the air pressure around it. This is so, because as the air pressure increases it compresses the balloon to a tighter space, which increases the pressure within, thus occurs an equilibrium on the balloon surface. However when you decrease the air pressure the balloon starts expanding to compensate which creates tension on the surface and pops it eventually.

The body is quite like a balloon, it is essentially a membrane enveloping a fluid, this holds even in the cellular level. But luckily for us liquids are much more sensitive to a change in their volume, meaning even the slightest compression is enough to create big pressures.

Long story short, the ocean squeezes the divers but since they have water inside their bodies, instead of air like in the case of balloons, the compression is not visibly noticable. Furthermore, try getting out of the water too fast and you will be dead within minutes.

DISCLAIMER: After reading my answer after a long time, I noticed it can misguide some to think that the reason we die when we get out of the water too fast is that it somehow ruptures us like popping a balloon. But this is not the case, I just thought that it is cool that they both get damaged by a decrease in pressure but not when by an increase.

• I think this answer could be usefully expanded to describe why bones aren't damaged either, even though they aren't fluids. The fact is that it's essentially impossible to permanently damage any material—solid, liquid, or gas—by applying a state of hydrostatic stress. The molecules are simply reversibly squeezed a little closer. – Chemomechanics Dec 29 '20 at 20:00

Divers can sustain periods of high ambient pressure as long as the internal pressure in their lungs and soft tissues can equalise with the external pressure. This is usually possible as long as the external pressure does not change too rapidly and they do not try to hold their breath. Problems may occur if air is trapped in internal spaces in the ear, sinuses or in tooth cavities - see this Wikipedia article - but divers are trained to recognise and handle these situations.

More dangerous than the actual pressure itself is decompression sickness, caused by dissolved gases in the blood stream coming out of solution as a diver ascends and creating bubbles which can block blood vessels and cause tissue damage.

The other answers are right--this is just to explicitly point out the reason that it feels worse when 30 people stand on your hand than when your body is subjected to an equivalent amount of hydrostatic pressure. The 30 people squash your hand like a ball of dough in a tortilla press, squeezing parts of it out the sides. Hydrostatic pressure, on the other hand, is homogenous, squeezing from all sides equally, and therefore not trying to change the shape of your body. It feels worse to turn your hand into a tortilla than to leave it as a hand.