Figure 17-15 A, The resting state of normal lungs when they are removed from the chest cavity; that is, elastic recoil causes total collapse. B, The resting state of a normal chest wall and diaphragm when the thoracic apex is open to the atmosphere and the thoracic contents are removed. C, The lung volume that exists at the end of expiration is the functional residual capacity (FRC). At FRC, the elastic forces of the lung and chest walls are equal and in opposite directions. The pleural surfaces link these two opposing forces. (Redrawn with modification from Shapiro BA, Harrison RA, Trout CA: The mechanics of ventilation. In Clinical Application of Respiratory Care, 3rd ed. Chicago, Year Book, 1985, p 57.)