Alveolar pressure

[1][2] During inhalation, the increased volume of alveoli as a result of lung expansion decreases the intra-alveolar pressure to a value below atmospheric pressure about -1 cmH2O.

This slight negative pressure is enough to move 500 ml of air into the lungs in the 2 seconds required for inspiration.

The lung alveoli collapse before air is expelled from them.

This forces the 500 ml of inspired air out of the lung during the 2–3 seconds of expiration.

By the end of expiration, the pressure drops gradually and becomes atmospheric again.