Fig. 2. Schematic drawing of the pressure–volume relationship of a single alveolus during inspiration and expiration (solid line).

At the start of the inspiration (A), the alveolus is collapsed. At point (B), the pressure increase has reached the critical opening pressure (Po), leading to an immediate volume increase (dashed line) as the alveolus is recruited (D). As the pressure is slowly decreased, there is little volume loss until the critical closing pressure (Pc) is reached at point (C). The alveolus immediately collapses to point (A). Note that Pc is lower than Po due to the law of Laplace.