Airway smooth muscle: the appendix of the lung.

It is well accepted that many organs in the human body no longer provide evolutionary advantage and appear to be lacking known function. These include the appendix, most body hair, wisdom teeth, male nipples, and external ear muscles, to name just a few. Even worse, many such organs, such as the appendix or wisdom teeth, can be the source of serious pain and medical problems. In this perspective, it is suggested that airway smooth muscle falls into this category—that of an organ with no essential physiologic function that can lead to serious medical problems. The function of smooth muscle in the airway wall has been speculated on for many years, but when looked at closely from a functional perspective, there really seems to be no need for airway smooth muscle. Indeed, most of the proposed reasons for presence of airway smooth muscle have not been based on experimental validation. In a recent discussion, Seow and Fredberg (1) commented that there is no known disease entity or physiologic deficit associated with loss of airway smooth muscle. They further suggested that perhaps airway smooth muscle was a vestigial remnant of its common embryologic origin with the gastrointestinal system, having no modern function. This conjecture, however, is in stark contrast to much classical thinking. In his extensive review article (2), Macklin could not imagine that such a system was there for no physiologic purpose, stating that “Organized as it is, into a very complex system, this muscle would seem of the utmost functional importance—in fact quite indispensable in respiration.” In this discussion, we consider the validity of 10 possible roles for airway smooth muscle that have appeared in the literature over the past 125 years. The emphasis will not be on whether smooth muscle is indispensable in respiration, but rather whether it is needed at all. These possible roles consist of the following: (1) peristalsis to assist exhalation, (2) peristalsis to assist mucus propulsion, (3) peristaltic contraction in the fetal lung to generate fluid pressure, (4) promoting lymphatic and venous flow, (5) ventilation/perfusion matching, (6) protecting the peripheral lung, (7) protecting airway structure, (8) stabilizing airways, (9) enhancing the effectiveness of cough, and (10) optimizing anatomic dead space volume.