If the outward pressure inside your body were not equal to the pressure pressing in on you, you would be crushed faster than an empty aluminum beer can at a frat party.

Find an old empty gallon can, a squarish one that paint thinner or turpentine comes in. Make sure its empty! Put an ounce or two of water in it. Put the can on the stove and heat it up until the water boils. Do this with the cap off!!! When the water boils, it will produce steam. When you see the steam coming out the top, screw the cap on. Don't burn yourself; use mitts. Let it cook for a few more seconds, not too long, though. Then, using oven mitts grab the can by the handle and quickly place it in the sink and run cold water over it. Watch what happens. Keep your hands away from it, though. You don't want to get pinched.

What happened? Well, when you quenched the can in the sink, the steam that had been filling the volume inside the can condensed, creating a vacuum -- an empty space -- inside the can. That created a low pressure in there. In other words, the pressure inside the can was much less then the pressure outside the can, creating a large pressure differential, causing the can to implode.

well this is simple i may be young and not know a lot but the fluids inside your body exert an equal amount of force all around you, thus acting on the atmospheric pressure and neutralizing the effect (some of this may not be correct)