Rapid alveolar liquid removal by a novel convective mechanism

Although alveoli clear liquid by active transport, the presence of surface-active material on the alveolar surface suggests that convective mechanisms for rapid liquid removal may exist. To determine such mechanisms, we held the isolated blood-perfused rat lung at a constant alveolar pressure (PA). Under videomicroscopy, we micropunctured a single alveolus to infuse saline or Ringer solution in approximately 10 adjacent alveoli. Infused alveoli were lost from view. However, as the infused liquid cleared, the alveoli reappeared and their diameters could be quantified. Hence the time-dependent determination of alveolar diameter provided a means for quantifying the time to complete liquid removal (C(t)) in single alveoli. All determinations were obtained at an PA of 5 cmH(2)O. C(t), which related inversely to alveolar diameter, averaged 4.5 s in alveoli with the fastest liquid removal. Injections of dye-stained liquid revealed that the liquid flowed from the injected alveoli to adjacent air-filled alveoli. Lung hyperinflations instituted by cycling PA between 5 and 15 cmH(2)O decreased C(t) by 50%. Chelation of intracellular Ca(2+) prolonged C(t) and abolished the inflation-induced enhancement of liquid removal. We conclude that when liquid is injected in a few alveoli, it rapidly flows to adjacent air-filled alveoli. The removal mechanisms are dependent on alveolar size, inflation, and intracellular Ca(2+). We speculate that removal of liquid from the alveolar surface is determined by the curvature and surface-active properties of the air-liquid interface.