Where does the respiratory zone begin in terms of airway generations?

• A. The terminal bronchioles mark the respiratory zone.
• B. The alveolar ducts mark the start of gas exchange.
• C. The respiratory bronchioles mark the start of gas exchange.
• D. The trachea marks the start of gas exchange.

Answer: (C)

The switch from conducting airways to the respiratory zone happens at the respiratory bronchioles. These bronchioles are the first airway segments where alveoli begin to appear along their walls, enabling gas exchange to start. Before that, at the trachea, bronchi, and terminal bronchioles, the airways are part of the conducting zone and lack alveoli, so no gas exchange occurs there. Gas exchange then continues in the alveolar ducts and sacs that follow. So, the respiratory zone begins with the respiratory bronchioles.