Mechanical factors in lung liquid distribution.

Abstract

The general principles of microvascular fluid exchange that apply to the lung are similar to those of all body organs. Fluid filtration across the pulmonary capillary membrane is governed by osmotic and fluid pressures of interstitial liquid and capillary blood according to the Starling equation. The degree of interstitial hydration results from a balance between the net filtration and the rate of clearance by lymphatics. Homeostasis in interstitial volume is maintained by compensations in osmotic and fluid pressure gradients and by changes in lymphatic absorption. These principles apply to the lung treated as a single homogeneous unit (38). However, the lung has unique properties with regard to fluid exchange because of its tissue-air interfaces and its ability to change volume with breathing. These properties result in a heterogeneous interstitium in which interstitial pressure is not uniformly distributed through­ out the lung. Because the lung is air-filled, its density is much less than that of a liquid, and the gravitational force results in vertical variations in lung expansion, blood flow and fluid filtration. These variations may affect regional lung water. The distribution of extravascular lung water is uniform throughout the normal lung because the lymphatic clearance matches the normal filtration. However, during abnormally high filtration, lymphatic clearance becomes inadequate and excess microvascular filtrate results in interstitial swelling, alveolar flooding, and eventually impaired gas exchange. This review focuses on the mechanical factors, such as interstitial pressure, resistance, and compliance, that determine the movement of liquid within the lung interstitium