Vulnerability of Pulmonary Capillaries During Exercise

John B. West

Disclosures

Exerc Sport Sci Rev. 2004;32(1) 

In This Article

Abstract and Introduction

Pulmonary capillaries have a dilemma. Their walls must be extremely thin for efficient gas exchange, but be immensely strong to resist the mechanical stresses that develop during heavy exercise. Elite human athletes at maximal exercise develop changes in the structure of the capillary wall as evidenced by red blood cells in their alveoli. Racehorses routinely break their pulmonary capillaries while galloping.

The wall of the pulmonary capillary has a basic dilemma. On the one hand it must have an enormous area and be extremely thin for efficient gas exchange by passive diffusion. For example, the total area of the blood-gas barrier (BGB) in the human lung is 50-100 m2 and over more than half of this enormous area, its thickness is only 0.2-0.3 µm.[2] But at the same time, recent work shows that the BGB must be immensely strong. The primary reason for this is that the hydrostatic pressure in the pulmonary capillaries rises to high levels during heavy exercise because of the large filling pressures demanded by the left ventricle to develop the necessary cardiac output.

This high capillary pressure results in a large hoop or circumferential stress in the wall of the capillary according to the Laplace relationship, and because the wall is so extremely thin, the mechanical stresses are enormous, approaching the breaking stress of the type IV collagen that is responsible for the strength of the wall. The result is that Thoroughbred racehorses routinely break their pulmonary capillaries while galloping. Elite human athletes at very high levels of exercise also apparently experience changes in the integrity of the BGB, as evidenced by the appearance of red blood cells and protein in their alveoli. This brief review covers the structure of the BGB, the development of mechanical stresses within it during exercise, the factors responsible for the strength of the BGB, the ultrastructural changes in experimental animals that accompany stress failure of the BGB, and a discussion of the evidence for changes in the barrier during heavy exercise in humans and also in racehorses.

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