Vulnerability of pulmonary capillaries during exercise.

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.