If we isolate the lung from the thorax and inflate or deflate it, we can study the relationship between lung volume and distending pressure. We obtain a curving line with a plateau: if we increase lung inflation beyond the plateau, the lung will rupture. The steepness of the slope is an index of the stiffness (elasticity) of the lung.
We can also perform the experiment with the thorax from which the lung has been removed. In this experiment the resting volume of the lung (that at which the transthoracic pressure difference is nil) is nearly 60% of the maximum volume of the lung (total lung capacity, TLC). We can make thoracic volume smaller by compressing the chest; a greater volume can be obtained by inflating it (or lower outside pressure). If the lung is in situ, then the thorax exerts a force on the lung, and vice versa, via the pleural space.
The illustration deals with someone aged about 20 yr. At about 35% of the TLC the elastic forces between thorax and lung balance: the thorax is below resting volume and pulls at the lung surface, lowering the pleural pressure sufficiently to inflate the lung to a volume at which elastic recoil balances the pull. This volume would be the functional residual capacity (FRC) if these were the only determinants of lung volume. At each lung volume we can add up the transthoracic and transpulmonary pressures, and thus obtain the pressure-volume characteristics of the respiratory system. This curve tells you what pressure needs to be applied to inflate the lung to a certain volume.