Strength of the pulmonary blood-gas barrier

The mammalian pulmonary blood-gas barrier is well known to be extremely thin. For example, in the human lung, half of the area of the barrier (the 'bulging' part) has a thickness of only 0.2-0.4 micron. We show here that the barrier is also immensely strong. This is an essential requirement because the capillary wall stresses during heavy exercise become very large (about 7 x 10(4) N/m2 = 70 kPa) when capillary pressure increases to 30 mmHg. Stress failure of the pulmonary capillary wall consistently occurs in experimental rabbit preparations at abnormally high pressures exceeding 40 mmHg and may be the cause of bleeding into the lung in galloping racehorses. The great strength of the thin side of the blood-gas barrier can be attributed to the extracellular matrix, especially the type IV collagen which is predominantly located in the very thin lamina densa. The alveolar wall is therefore particularly vulnerable to injurious agents which attack type IV collagen such as autoantibodies in Goodpasture's Syndrome and perhaps neutrophil elastase in emphysema. The combination of extreme thinness and great strength of the blood-gas barrier poses a unique design requirement.