Bronchioles: Terminal vs. Respiratory—Structure, Function & the Transition to Gas Exchange

Bronchioles are the final conducting airways of the lung, measuring less than 2 mm in diameter. They come in two distinct types, each with a unique structure and role in the journey from air conduction to gas exchange.

Terminal Bronchioles

• Smallest purely conducting airways—no gas exchange occurs here.
• Lined by ciliated simple cuboidal epithelium with club (Clara) cells.
• No alveoli in their walls.
• Control airflow distribution via smooth muscle tone (constriction/dilation).

Respiratory Bronchioles

• Distal to terminal bronchioles; they have scattered alveolar outpouchings budding from their walls.
• Gas exchange begins here.
• Lined by ciliated cells, club cells, and type I/II pneumocytes in the alveolar outpouchings.
• Club cells secrete surfactant components and detoxify inhaled substances.

The Big Picture

Together, terminal and respiratory bronchioles form the critical transition zone. Terminal bronchioles deliver air; respiratory bronchioles initiate the oxygen-carbon dioxide exchange that sustains life. This structural progression protects the delicate alveoli downstream and optimizes ventilation-perfusion matching—ensuring air and blood meet where it matters most.