Which portion of the respiratory tract conducts air but does not participate in gas exchange?

• A. Respiratory zone
• B. Conducting zone
• C. Alveolar sacs
• D. Alveolar ducts

Answer: (B)

The important idea is separating air-conducting pathways from gas-exchange regions. The conducting zone moves air to the lungs and conditioned it—warming, humidifying, and filtering—without participating in gas exchange, because it lacks the many alveoli and dense capillary networks needed for transfer of O2 and CO2. The respiratory zone contains the alveoli and surrounding structures where gas exchange actually occurs across the thin alveolar walls and capillary membranes. So the portion that conducts air but does not engage in gas exchange is the conducting zone, which includes the trachea, bronchi, and bronchioles up to the terminal bronchioles. Gas exchange takes place downstream in the alveolar-rich regions.