Breathing provides the oxygen needed for the conversion of foodstuff to the energy used to support and maintain the various functions of life. The intake of oxygen is facilitated by the lungs.

Breathing is vital for life. Humans can live only a very short time without an infusion of oxygen. Without oxygen, irreversible alterations in the brain set in after only about 4-5 minutes. Reflecting this importance to life, breathing is an automatic function, and is controlled by nerve cells in the brain stem. Signals from the brain are sent down the spinal cord via the phrenic nerve to two sets of muscles that control the operation of the the diaphragm, a sheet-like muscle located at the bottom of the chest cavity, that moves down to force air into the lungs and up to force air out of the lungs.

Air can be breathed into the lungs through...