ABSTRACT
It is now commonly understood that surface tension forces within the thin layer of lining uid that coats the respiratory airways and alveoli are fundamental to the mechanical characteristics of the lung. To illustrate this, Figure 10.1 shows the pressure-volume relationship of one lung inated and deated with air and the other with saline (Neergaard 1929). e air-inated lung demonstrates that for any volume; a substantially higher pressure is required during ination as compared to deation. In contrast, the ination and deation limbs of the saline-lled lung are nearly coincident. When a system behaves dierently during the application than it does during the removal of a force, it is said to have a hysteresis. e lower pressures and small hysteresis area of the liquid-lled lung in comparison to the air-lled lung demonstrates that surface tension forces provide the lung with the majority of its elastic recoil.
