Mechanics of lung fluid balance.

Recent research in pulmonary physiology, anatomy, and mechanics have clarified our general understanding of liquid and solute transport through the lung. Fluid crosses the microvascular endothelial membrane at a rate that depends on gradients in the transmembrane hydrostatic and osmotic pressures and the conductance of the permeable membrane. Under normal conditions, the filtered fluid is removed by an efficient lymphatic pump. Edema accumulates in the lung when an increased flux due to an elevated vascular pressure or to a more permeable membrane is not matched by an adequate lymph clearance rate. Initially a favorable hydrostatic pressure gradient drives the excess fluid into interstitial spaces surrounding large blood vessels and airways away from filtration sites near capillaries and thereby ensures efficient gas exchange. Further edema formation reduces the pressure gradient, eventually leading to the flooding of alveolar air spaces and impaired gas exchange. I will focus on the role of the above forces in the regulation of extravascular lung water. It will become clear that many details of the general scheme are not known, and our conceptual understanding of the relevant mechanisms involved is often rudimentary and incomplete. Some of the more important questions pertain to the interstitial pressure around capillaries, the resistance and compliance of the interstitial matrix, and the role of the lymphatics in regulating interstitial fluid volume and interstitial pressure.