Commentaries on viewpoint: unresolved mysteries.

TO THE EDITOR: Mysteries remain unresolved if the approach is misconceived. In this Viewpoint (3) alveoli are considered as structures sui generis—but they are not. Alveoli are rather air pockets within the ingenious architecture of a fiber continuum designed to expose capillary networks over a large surface to air refreshed from alveolar ducts (4). This fiber continuum spans from the peripheral fiber net in the pleura through the alveolar septa, where the fine fibers are interwoven with the capillary network, to the axial fiber network of alveolar entrance rings that forms the “wall” of the alveolar ducts (5). Alveoli pop open at the entrance rings when the fiber continuum is increasingly tensed upon inflation, a process governed by surface forces active at the air-tissue interface and modulated by surfactant (5). In— here disregarded—studies Bachofen et al. (1) have shown that the alveolar surface area increases steeply when deflated lungs are gradually inflated to TLC, thus confirming the observation of Hajari et al. (2). But that is not the end of the story: when these inflated lungs are deflated-inflated between 80% and 40% TLC (the breathing range in exercise) the surface area changes by a mere 20% for a factor 2 change of air volume (1). This is the result of surfactant-modulated surface forces that are effective as we breathe along the deflation limb of the pressure-volume curve (1, 5). The “mysteries” are resolved if we consider not alveoli but the micromechanics of the interaction between fiber tension and surface forces in “making alveoli.”