Our bodies are strong enough to resist the forces that compress us.A good analogy is blowing up a balloon. When the balloon is small even though the pressure of air inside the balloon exceeds the surrounding pressure, the strength of the elastic balloon is great enough to support the difference in pressure. but if one keeps blowing up the balloon, the elastic membrane (i.e. the balloon) gets thinner and thinner and the pressure difference gets bigger and bigger until the elastic strength of the balloon is exceeded and...POP!!!!

You can test this theory by buying balloons of different skin thickness and seeing which one pops first. The maximum size before popping should correlate with the thickness of the balloon skin measured before blowing up. You will need to use a micrometer to measure the thickness of the balloon.

It is because we have evolved so that our internal pressure matches the atmospheric pressure, so we are in equilibrium. When you go up in an airplane, your ears tend to hurt until you can get them to pop because, at that altitude, your internal pressure is greater than the external pressure, so until you can get your ears to "pop" - that is, your internal pressure to equalize with the external pressure, you feel pain.

Now, because physicists always like to think of what happens in the extreme, let's ask: knowing that your internal pressure is equalized by evolution to atmospheric pressure of 14.7 pounds / square inch, what would happen if:

1) You go into space without a space suit? - you would stretch out like a balloon, due to the elasticity of your skin; then you would burst due to the fact that your internal pressure is 14.7 pounds/square inch but the outside pressure is ZERO!And then all the liquid in your body would instantly freeze. YUCH!!

2) What would happen if you go to the bottom of the deepest part of the ocean without a diving bell? You guessed it: you would implode due to the excessive external pressure!

We are being pressed equally from all directions,including internally. The air inside of your lungs is at the same pressure as the air outside, so it presses your chest cavity outward with the almost same force as it is being pressed inward (slightly less, because there is more surface area on which pressure is applied outside than in, and that is why you exhale when you relax).

As for the rest of your body, it's mainly made up of water, which is a liquid and not a gas. Liquids are incompressible: they push back with the same pressure that is pressed upon them by themselves. If you were to be taken to the bottom of the ocean, your lungs would implode because the air inside of them would not have enough pressure to hold up the volume of water that is above you, but your head would not get squashed because it's made of liquid and solid, and they don't squash under uniform pressure. They only deform under non-uniform pressure (there has to be somewhere for squashed solids or liquids to go or they won't squash).

This is a bit oversimplified, but it gives you the right general idea...Our skin and organs is mostly fluid (water) or solid (bone), and neither of these is compressible. Our lungs are filled with air, but it's at the same pressure which the surrounding atmosphere has (760 Torr, or 14 PSI). If you were to pump the air out of your lungs, so there was less pressure inside your body than outside, then your lungs would collapse. (Even before that, you would suffer from bleeding into the lungs, and a severe case of "the bends" as nitrogen in your bloodstream suddenly evaporated and formed bubbles, blocking blood flow. But those are different effects from the simple collapsing you asked about.)

It's true that the weight of the column of air above us is very heavy. We can call this weight an external pressure, because it is pushing down on us. However, the reason we, nor other objects, are crushed by the weight of this air is because this external pressure is balanced by our internal pressure, which arises from various fluids and materials we are composed of. This internal pressure exists because we are largely made up of incompressible solids and liquids. To simplify matters, just think about a balloon filled with water. The external pressure is trying to compress the balloon, while the water inside the balloon is able to balance this external pressure due to the relative incompressibility of water. This incompressibility is responsible for the internal pressure opposing the external pressure.