The atmosphere exerts a pressure of 14.7 pounds per square inch on everything, including our bodies. This pressure is due to the gravitational force that the Earth exerts on the atmosphere: a column of air with cross-sectional area of one square inch, between here and outer space, weighs 14.7 pounds.
This pressure is exerted in all directions simultaneously; if it were not, the air would always be "falling" or moving in some other direction. When you consider the entire surface area of your body, the collective inward force is about ten tons! Why aren't you crushed by the atmosphere?
If you took all of the air out of a sealed steel drum, it would be gradually crushed before your eyes as if it were in a giant trash compactor. This demonstration is sometimes used in science "magic" shows. It is the air within our bodies and all of the objects around us that keeps them from collapsing under the weight of the atmosphere. We are in constant equilibrium with the atmosphere.
Let's consider one situation where the human body is taken out of equilibrium with the atmosphere: when you are under water, scuba diving or snorkeling. The reason why you expand your chest when you breathe is that you have to reduce the air pressure inside your lungs to get air to flow into them. When you are under water, it is hard to expand your chest because the inward pressure on your body is much greater than normal; water weighs so much more than an equivalent volume of air. Under ten feet of water, the inward pressure is about 30% greater than normal; this is enough to prevent you from adequately expanding your chest in order to breathe through a snorkel. To get around this problem, scuba divers use air from a pressurized tank for breathing; this air literally forces its way into your body and expands your lungs for you.
If you go scuba diving very deep down, you must use very high pressure air to breathe. After a while, the pressure of the dissolved air in every fluid in your body is increased to match the pressure of the air you are breathing; your body becomes like a sealed bottle of soda: there are no bubbles because the gas is dissolved in an environment of pressure equilibrium. If you come back to the surface too quickly, your body is out of equilibrium with normal atmospheric pressure; like in a soda bottle just opened to the atmosphere, bubbles of air would come out of solution in your body and accumulate in various places. If the bubbles accumulate in the bloodstream, death results. If the bubbles gather in body cavities and open spaces, such as the joints, the painful condition called "the bends" results.