Smallairways

Small airways are the distal part of the airway tree, generally defined as airways with an internal diameter of less than about 2 millimeters. They lie distal to the terminal bronchioles and include small bronchioles and the proximal portions of the respiratory bronchioles. Their walls lack cartilage and contain relatively little submucosal tissue compared with larger airways. The epithelium becomes thinner, goblet cells are scarce, and club cells are more common; smooth muscle is present and can constrict in response to stimuli.

Anatomy and function of the small airways contribute to ventilation and airway resistance. Because of their small diameter, these airways collectively contribute to total airway resistance, particularly when inflamed or obstructed. They also participate in humidification and mucociliary clearance, though most gas exchange occurs beyond them in the respiratory zones. In healthy breathing, a larger share of resistance is in the larger airways, but obstruction in the small airways can significantly affect ventilation, especially during expiration, and can lead to air trapping.

Clinical significance is most evident in small-airways disease, a pattern of pathology seen in chronic obstructive pulmonary disease, asthma, bronchiolitis, cystic fibrosis, and after certain infections or inhalational exposures. The condition features inflammation, luminal obstruction by mucus or debris, and remodeling of the airway walls, which narrows diameter and increases work of breathing and residual volume. Small-airways disease can be difficult to detect with routine imaging because changes occur in the distal airways.

Diagnosis and monitoring rely on specialized physiological and imaging approaches. Reduced mid-expiratory flow (FEF25-75) on spirometry and impedance or oscillometry indices can indicate small-airway dysfunction; expiratory CT may reveal air trapping; bronchoscopy typically assesses proximal airways but can aid in ruling out other causes. Treatment targets the underlying disease and may include anti-inflammatory therapies and bronchodilators, with emphasis on minimizing exposure to injurious agents and optimizing overall lung function.