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Breathing is due to changes of pressure within the thorax in comparison with the outside.
Air moves from a region of higher pressure to one of lower pressure.
Conventions
When the mechanics of breathing are being discussed, atmospheric pressure is conventionally referred to as 0 cm H2O, so lowering alveolar pressure below atmospheric pressure is known as negative-pressure breathing.
As soon as a pressure difference sufficient to overcome the resistance to airflow offered by the conducting airways is established between the atmosphere and the alveoli, air flows into the lungs.
It is also possible to cause air to flow into the lungs by raising the pressure at the nose and mouth above alveolar pressure.
This positive-pressure ventilation is generally used on patients unable to generate a sufficient pressure difference between the atmosphere and the alveoli by normal negative-pressure breathing.
Air flows out of the lungs when alveolar pressure is sufficiently greater than atmospheric pressure to overcome the resistance to airflow offered by the conducting airways.