Mechanical factors determining pulmonary interstitial fluid pressure.

Abstract

The pulmonary interstitium extends from sites adjacent to capillaries to the loose connective tissue around bronchi and large blood vessels. The interstitial fluid pressure in different parts of the interstitium depends primarily on the surface forces acting on adjacent structural members. At the alveolar level, the surface pressure is the alveolar gas pressure, which is modified by the alveolar surface tension in edematous lungs. In the perivascular interstitium around bronchi and large blood vessels, the surface pressure results from the balance between nonuniform stress in the parenchyma lying on one side of the interstitium and the elastic recoil of the vessel wall attached to the other side. Perivascular interstitial edema increases the interstitial fluid pressure by decreasing the nonuniform parenchymal stress and recoil of the vessel wall. A vertical gradient in interstitial pressure due to the interstitial fluid acting as a hydrostatic column is absent even in edematous excised lungs, because the surface forces determine the interstitial pressure locally and the resistance to interstitial fluid flow is relatively large. In the intact chest, vertical gradients in lung static recoil can result in significant vertical gradients in perivascular interstitial fluid pressure. During edema formation, the interstitial pressures are different in the two interstitial compartments, establishing a pressure gradient that tends to move fluid from the septal to perivascular interstitium.