A perspective of respiratory mechanics.

Breathing was recognized very early to be a muscular action. The participation of the diaphragm, intercostals, and accessory muscles was appreciated by Galen. Consideration of a possible role for smooth muscle in breathing did not occur until much later. Even today smooth muscle is seldom included as a topic in discussions of "respiratory mechanics." Bronchial smooth muscle was first described in the classic study of Reisseisen in the early 19th century, although the presence of contractile elements in lungs had been demonstrated a few decades previously. An important comprehensive investigation of the action of bronchial smooth muscle was published in 1892 by Willem Einthoven. His experimental approach became a paradigm. On the other hand, his analysis of dynamic collapse of the airways received little attention and was independently arrived at half a century later. Although we now have a considerable understanding of the mechanics of bronchial smooth muscle and of the effects of numerous physiological and pharmacological agents on its behavior, the exact role it plays in normal physiological function is unclear. Numerous plausible suggestions have been made, but none has been convincingly demonstrated.