<P> 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). </P> <P> During expiration, the opposite change occurs . 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 . </P>

Why does the pressure in the alveoli return to zero at the end of inspiration