Alveolar pressure

Alveolar pressure (P_alv) is the pressure of air inside the lung alveoli. When the glottis is opened and no air is flowing into or out of the lungs, alveolar pressure is equal atmospheric pressure (zero cmH2O).

Significance

During inspiration, the increased volume of alveoli as a result of lung expansion decrease 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 2 seconds required for inspiration. At the end of inspiration, the alveolar pressure returns to atmospheric pressure (zero cmH2O).

During expiration, opposite change occure. The lung alveoli collapse before air is expelled from them, The alveolar pressure rises to about +1 cm H2O. This forces the 500 ml of inspired air out of the lung during 2-3 seconds of expiration. By the end of expiration, the pressure drops gradually and become atmospheric again.