Lung compliance is determined from the slope of the pressure-volume curve during deflation.
Lung distensibility during inflation is low compared to its distensibility during deflation. The main reason is the variable air-liquid surface tension in the alveoli, which have a very small radius. At end-expiration the lungs recoil to a smaller volume, but the chest wall recoils in the opposite direction toward a larger volume. Thus the chest wall is normally under compression in the range of tidal breathing. Because the lungs and chest wall move together the total compliance of the respiratory system is less than that of either part alone.