ABSTRACT
A basement membrane separates the pulmonary capillary endothelial cells from the interstitial space. Pulmonary capillaries tend to have a continuous endothelium and appear to be more permeable than muscular capillaries. The interstitial compartment includes the area in continuity with the basement membranes of the alveolar epithelial cells and pulmonary capillaries as well as the region proximal to the alveolar ducts. The movement of pulmonary fluid and solutes is influenced by physical forces acting on the anatomic structures and by the permeability characteristics of the membranes. Increases in alveolar pressures will increase interstitial tissue pressure and pulmonary capillary hydrostatic pressure, so little fluid movement across the pulmonary capillaries occurs. The capability of the pulmonary lymph system to remove fluid from the lung is one safety factor opposing edema formation. The pulmonary lymphatics may play an important role as one of several factors involved in the process of fluid removal and fluid retention in the lungs.
