Bronchiole: small airways of the lung

Structure and types

Bronchioles are classified by size and function. Terminal bronchioles are the smallest purely conducting airways; distal to them are respiratory bronchioles that have occasional alveolar outpocketings and participate in gas exchange. Histologically, bronchioles have a lining of ciliated epithelium, goblet or club cells, and a variable layer of smooth muscle and connective tissue.

Function and physiology

Bronchioles regulate airflow distribution through smooth muscle contraction and dilation, which affects airway resistance and ventilation. Their epithelium helps clear inhaled particles with cilia and secretes substances that protect airway surfaces. The transition from respiratory bronchioles to alveolar ducts marks the shift to efficient gas exchange.

Development and adaptation

Bronchioles form during lung branching morphogenesis in the embryo as distal extensions of the bronchial tree. They continue to mature after birth as alveoli increase in number and surface area. Bronchioles can change caliber in response to autonomic signals, inflammation, or injury.

Clinical importance

Disease of the bronchioles contributes to common respiratory conditions. In asthma, bronchoconstriction of bronchioles increases resistance and causes wheeze. Viral bronchiolitis mainly affects infants and inflames terminal airways. Chronic bronchiolitis and bronchiolitis obliterans involve persistent inflammation and narrowing that impair ventilation.

Notable distinctions

• Bronchi vs bronchioles: bronchi have cartilage and larger calibre; bronchioles do not.
• Terminal bronchioles are conducting; respiratory bronchioles participate in gas exchange.
• Small changes in bronchiole diameter can have a large effect on airflow and symptoms.

For general background on lung anatomy and airway disease, see resources linked to the terms above (lungs, cartilage, alveoli).