Ultrastructural aspects of pulmonary edema.

The alveolar septa are designed for gas exchange as well as for the exchange of water and solutes. The organization of the tissue elements and the ultrastructural architecture of the alveolar septa minimize the interference between gas and liquid exchange even when excess water begins to accumulate in the septal interstices and the alveoli. Larger amounts of alveolar edema fluid, however, profoundly alter both function and structure of the lung parenchyma. As to the routes of abnormal fluid movement, the ultrastructural findings are not always conclusive, though in some types of permeability edema frank anatomic disruptions of the endothelial and epithelial barriers can be observed. On the other hand, in hemodynamic edema the permeability barriers appear to be intact. However, experiments involving macromolecular tracers and the presence of numerous red blood cells in the interstitial and alveolar spaces indicate that transient separations of cell junctions have taken place.