[Why are the lungs dry?]

The extremely thin blood-gas barrier, the high blood perfusion rate and the deformability of the lung required for ventilation call for safety measures in order to keep the peripheral airspaces dry. The protective factors are provided in part by the particular structural organization of the lung, in part by physiological safeguards. Amongst the structural safety factors the extremely low permeability of the alveolar epithelial cell layer, the effective drainage system of interstitial spaces, and the loose connective tissue layers which surround vessels and bronchi, and which can act as transient fluid reservoirs, should be mentioned. The physiologic safety factors include the low hemodynamic pressures in the pulmonary vessels, the high colloid-osmotic pressure of blood, the decrease in perimicrovascular colloid-osmotic pressure on increased transcapillary fluid filtration, the interstitial pressure gradient between peripheral and central parts of the lung, and the minimal mechanical forces acting on the fine lung parenchyma owing to the low surface tensions provided by alveolar surfactant. Whether the active pumping mechanism improving reabsorption of edema fluid is also operative under normal conditions has not yet been clarified.