Compliance is the ability of lungs and thorax to expand. Lung compliance is defined as the volume change per unit of pressure change across the lung. Measurements of lung volume obtained during the controlled inflation/deflation of a normal lung show that the volumes obtained during deflation exceed those during inflation, at a given pressure.
C=V/P
Compliance describes the elastic properties of various parts of the respiratory system. Compliance represents a volume change per unit change in pressure (200 ml/cm H2O in the normal lung). The total respiratory compliance consists of combined lung and chest wall compliance and is normally 70–80 ml/cm H2O.