It is remarkable that it has taken so long to recognise how vulnerable the pulmonary capillaries are to mechanical failure during strenuous exercise. After all, the extreme thinness of the capillary wall was appreciated when the first electron micrographs were obtained by Low in 1952. We now know that the total area of the blood-gas barrier (BGB) in the human lung is 50–100 m² and that for more than half of this enormous area the thickness is only 0.2–0.3 μm.¹ The result is that during severe exercise when the pulmonary vascular pressures rise to high levels, the capillary wall stresses become extremely high approaching the breaking stress of collagen.² It is not surprising therefore that changes in the integrity of the BGB occur under these conditions.

Perhaps investigators were initially misled by early data from cardiac catheterisation procedures which suggested …