Q. Why do you not collapse in the pressure created by the atmosphere?

Answer:-

If the outward pressure inside your body weren't equal to the pressure pressing in on you and you would be crushed faster than an empty aluminium beer can at a frat party.

Find an old unfilled gallon cans a squarish one that paint thinner or turpentine comes in. Ensure that it is empty! Put an ounce or two of water in it. Put the can on the stove also heat it up until the water boils. Do these without the cap when the water boils it will produce steam. When you observe the steam coming out the top put the cap on. Do not burns up yourself use mitts. Let it cook for some more seconds not too long though. After that utilizing oven mitts grab the can by the handle and speedily place it in the sink and run cold water over it. Watch what takes place. Keep your hands apart from it though. You do not want to get pinched.

What take placed? Well when you reduce 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 formed a low pressure in there. In other sense 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 easy 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).