You breathe with the help of your diaphragm and other

muscles in your chest and abdomen. These muscles literally

change the space and pressure inside your body to

accomodate breathing. When your diaphragm pulls down, it

not only leaves more space for the lungs to expand but also

lowers the internal air pressure. Outside, where the air

pressure is greater, you suck in air in an inhale. The air

then expands your lungs like a pair of balloons. When your

diaphragm relaxes, the cavity inside your body gets smaller

again. Your muscles squeeze your rib cage and your lungs

begin to collapse as the air is pushed up and out your body

in an exhale.