The soap-bubble model of alveolar behavior in lung expansion is obsolete, but no perfect other model exists. It is unclear whether the lung inflates by alveolar enlarge- ment or by alveolar recruitment and unfolding. Also unclear is the role of surfactant in maintaining alveolar stability, especially as its ability to reduce surface tension re- quires spherical alveoli, whereas alveoli are polyhedral. Our studies suggest that alveoli do not enlarge isotropically and that surfactant does play a role in their stability. The interdependence of alveoli also plays a role in stabilization, and most likely both surfactant and interdependence are necessary for stability. The most important clinical implication is that an intermediate stage of atelectasis exists during which alveoli are unstable but not totally collapsed, which reduces gas-exchange efficiency and produces V/Q imbalance. However, detection of this stage by most noninvasive techniques is impossible, and more study is necessary to learn why this is so.
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