Current concepts of lung-fluid balance

The maintenance of pulmonary fluid homeostatis is dependent on a complex relationship between the amount of fluid filtered by the capillaries and that drained by the lymphatics. The capillary endothelium has a relatively high permeability to plasma proteins, whereas the normal alveolar epithelium is totally impermeable to protein, preventing it from entering the alveolar space. Fluid and protein sieved by the capillary endothelium flows along the parenchymatous connective tissue and is drained by the juxta-alveolar lymphatics. The amount of transvascular fluid and protein flux is determined by the hydrostatic and oncotic forces in the capillaries and interstitial space and by the permeability of the epithelium. Pulmonary edema can occur if any of these factors become sufficiently altered. However, the lung is highly resistant to excess fluid accumulation, thanks to a number of edema safety factors. Finally, it has been demonstrated that low alveolar surface tension is important not only to lung mechanics but also to the maintenance of normal lung-fluid balance.