STRESS-INDUCED INJURY OF PULMONARY CAPILLARIES

The lung is particularly vulnerable to injury because the blood-gas ba rrier is so extremely thin. Furthermore, the mechanical stresses in th e barrier become very high when capillary pressure is raised, or when the lung is inflated to a high volume. The strength of the blood-gas b arrier on the thin side can be attributed to the type IV collagen in t he basement membranes. Abnormally high stresses in the walls of the pu lmonary capillaries result in ultrastructural changes including disrup tions of both the alveolar epithelial and capillary endothelial layers . All Thoroughbred racehorses break their pulmonary capillaries when t hey gallop. Also, elite human athletes develop changes in the permeabi lity of the blood-gas barrier at high levels of exercise. Pathological conditions resulting in stress failure include: 1) high-altitude pulm onary edema; 2) neurogenic pulmonary edema; 3) severe left ventricular failure; 4) mitral stenosis; and 5) overinflation of the lung. There is a spectrum of low permeability to high permeability edema as the ca pillary pressure is raised. Remodeling of pulmonary capillaries appare ntly occurs at high capillary pressures. It is likely that the extrace llular matrix of the capillaries is continuously regulated in response to capillary wall stress.