Therefore, the only unknown quantity in this expression is V 2 which is the sum of the bag and lung volumes. Subtracting V 1 from V 2 then gives lung volume. A body plethysmograph is essentially a large closed box in which a subject sits. Mouth pressure equals alveolar pressure because there is no flow along the airways. This can be used to used to estimate absolute lung volume quite accurately.
P is the pressure required to push it.
Pulmonary compliance is obtained if P is the change in transpulmonary pressure , Ptp, defined as provided that there is no flow along the airways so that This is achieved in practice by inflating the lungs with a known V and then holding lung volume constant while Ptp is measured and related to the value of Ptp prior to inflation. The pressure-volume curve also exhibits hysteresis . The term hysteresis relates to the difference between the inflation and deflation PV curves. It represents the extra energy that is required to inflate the lung compared to deflating it. This doesn t mean that the alveoli have collapsed, nor that all the airways have closed.