Bronchioles: Anatomy, Function, and Role in Lung Defense

Bronchioles are the small airways (≤2 mm diameter) that branch from the bronchi and terminate in alveolar ducts and sacs. They are essential for air conduction, gas exchange, airflow regulation, and pulmonary defense.

Structure and Function

• Conducting bronchioles: No alveoli; lined by ciliated cuboidal epithelium (club cells); no cartilage or glands.
• Respiratory bronchioles: Have scattered alveoli; begin gas exchange.
• Terminal bronchioles: Last purely conducting generation; mark the end of the conducting zone.

Key Functions

1. Air conduction: Low-resistance pathway for airflow to alveoli.
2. Gas exchange: Respiratory bronchioles and alveolar ducts allow O₂/CO₂ diffusion.
3. Airflow regulation: Smooth muscle in bronchiolar walls enables bronchodilation (sympathetic/β₂) and bronchoconstriction (parasympathetic, histamine, cold air). Critical in asthma/COPD.
4. Defense: Mucociliary escalator traps and clears particles/pathogens; club cells secrete surfactant proteins and detoxify oxidants.
5. Immune surveillance: Intraepithelial lymphocytes, macrophages, and dendritic cells respond to inhaled antigens.

Bronchiolar disease (bronchiolitis, asthma, bronchiolitis obliterans) impairs ventilation and gas exchange. Understanding their physiology guides management of obstructive lung diseases.