Compliance refers to the distensibility of an elastic structure (such as the lung) and is defined as the change in volume of that structure produced by a change in pressure across the structure. It is important to understand that the lung (or any other elastic structure) will not increase in size if the pressure within it and around it are increased equally at the same time.
In a normal healthy lung at low volume, relatively little negative pressure outside (or positive pressure inside) needs to be applied to blow up the lung quite a bit. However lung compliance decreases with increasing volume. Thereforeas the lung increases in size, more pressure must be applied to get the same increase in volume. This can be seen from the following pressure-volume curve of the lung: Lung compliance and the slope are the same:
Compliance can also change in various disease states. For example, in fibrosis the lungs become stiff, making a large pressure necessary to maintain a moderate volume. Such lungs would be considered poorly compliant. However, in emphysema, where many alveolar walls are lost, the lungs become so loose and floppy that onlya small pressure difference is necessary to maintain a large volume. Thus, the lungs in emphysema would be considered highly compliant.