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 ∼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 cmH₂O. 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 airfilled alveoli. Lung hyperinflations instituted by cycling PA between 5 and 15 cmH₂O decreased C_t by 50%. Chelation of intracellular Ca²⁺ prolonged C_t and abolished the inflationinduced 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 Ca²⁺. We speculate that removal of liquid from the alveolar surface is determined by the curvature and surface-active properties of the air-liquid interface.