Bronchioles: The Small Airways Where Obstructive Lung Disease Often Begins Silently

What Are Bronchioles?

Bronchioles are the smallest conducting airways (<2 mm diameter), branching from segmental bronchi and terminating in alveolar ducts/sacs.

Histology

• Ciliated simple cuboidal epithelium (no cartilage, no glands).
• Clara (club) cells – secrete surfactant components, detoxify.
• Smooth muscle layer – allows bronchoconstriction/dilation.
• No goblet cells beyond terminal bronchioles.

Functional Zones

• Conducting bronchioles (terminal): No gas exchange.
• Respiratory bronchioles: Alveoli bud from walls; gas exchange begins.
• Alveolar ducts/sacs: Primary gas exchange units.

Physiological Role

• Distribute air evenly.
• Regulate airflow resistance via smooth muscle tone (autonomic control).
• Mucociliary clearance (cilia + Clara cell secretions).

Clinical Significance

• Asthma: Bronchiolar smooth muscle hyperreactivity, inflammation, mucus plugging.
• Bronchiolitis (infants): RSV → edema, necrosis, obstruction.
• COPD: Small airway disease (bronchiolitis obliterans) precedes emphysema.
• Bronchiolitis obliterans: Post‑transplant, chemical injury, connective tissue disease.

Bronchioles are the "quiet zone" where early obstructive pathology often starts silently.