Factors determining airway caliber in asthma.

Airway hyperresponsiveness is a hallmark of asthma in which airways narrow excessively in response to an agonist, resulting in difficulty in breathing. Constriction of the smooth muscle that spirals around the airways is the principle cause of airway narrowing during an asthma attack. It is likely that several mechanisms are involved in the development of a hyperresponsive airway in asthma. In this review, we focus on the structural and functional aspects that govern the narrowing of a single airway within a lung, then we review the current understanding of how these factors become altered in a way that leads to the airway hyperresponsiveness observed in asthma. We first examine airway caliber as a simple equilibrium of forces favoring narrowing and the forces opposing this narrowing. We then review the role that the dynamic forces of tidal breathing and deep inspirations have across all length scales of the respiratory system; we describe an intriguing inconsistency that has arisen from these data. Finally, we examine the interaction between airway remodeling and inflammation and their roles in health and disease.