Only in the past 15 years has it been appreciated that pulmonary capillaries are remarkably mechanically fragile, and that ultrastructural changes can occur in their walls under normal though unusual physiological conditions. We now know that many pulmonary and cardiovascular disorders are associated with structural failure of the walls of the capillaries in response to high mechanical stresses ("stress failure"). In retrospect it is extraordinary that it took so long to realize the vulnerability of pulmonary capillaries because recognition of the extreme thinness of their walls goes back to the first electron micrographs in 1952. A simple calculation on the back of an envelope using the Laplace relationship will show that any substantial increase in transmural pressure of the capillaries must result in extremely high hoop or circumferential stresses. Perhaps people were misled by early measurements suggesting that pulmonary capillary pressures remained low under all physiological conditions, although we now know that this is a serious misconception. This brief account summarizes information on the structure and strength of pulmonary capillaries, the stresses that can develop in their walls under both physiological and pathological conditions, the phenomenon of stress failure, and recent work on how the structure of the capillary wall is regulated.

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