I control the pressure in my upper body whenever I feel like breathing ...
an urge that typically grabs me several times each day.
-- When I want to inhale, I move my diaphragm down. This expands my
chest cavity, reducing the pressure in it, and atmospheric pressure then
causes outside air to pour into my lungs.
-- When I want to exhale, I move my diaphragm up. This contracts my
chest cavity, increasing the pressure in it, and the compressed air in my
lungs then pours out, into the lower pressure of the surrounding atmosphere.