The significance of alveolar geometry and surface tension in the respiratory mechanics of the lung.

It is customary to describe alveolar respiratory mechanics in terms of a bubble-shaped model alveolus, using the Laplace equation. The coexistence of alveoli of different radii cannot be satisfactorily explained by this model, even if hypotheses as yet unconfirmed by experiment are taken into account. Moreover, this model requires the assumption of extremely low surface tensions, near 0 dny/cm, to explain the absence of atelectasis with very small alveolar radii and the maintenance of alveolar fluid balance. On the basis of investigations of the dynamic surface tension of lung alveolar surfactant from rat lungs, however, the minimal surface tension of alveoli is 18-20 dny/cm. In addition, the lung does not consist of isolated and spherical alveoli but a dense packing of polyhedral air spaces separated by alveolar septa. This paper is an attempt to analyze the recoil forces of the lung due to surface tension on the basis of the polyhedral alveolar structure and the minimum surface tension mentioned above. It is demonstrated that several geometrical parameters are able to guarantee the stability of the alveolar structure of the lung to a greater extent than a variable surface tension. It is concluded that not a single (and fictive) alveolus but the septal intersections and the "peripheral" septa (limiting only one air space) are the smallest morphological units of the respiratory mechanics. Some consequences concerning the pressure-volume behavior of the whole lung, the above mentioned coexistence problem and the alveolar fluid balance are discussed.