Which pressure difference produces airflow in the conducting airways?

• A. Transrespiratory Pressure
• B. Transairway Pressure
• C. Transpulmonary Pressure
• D. Transthoracic Pressure

Answer: (B)

Airflow through the conducting airways is driven by the pressure difference across those airways—from the mouth opening to the alveolar space. This gradient is called transairway pressure. When you breathe in, the pressure at the mouth is roughly atmospheric while the pressure inside the alveoli drops as the lungs inflate, so the transairway pressure (mouth pressure minus alveolar pressure) becomes positive and pushes air into the lungs.

This specific gradient across the conducting airways is what moves gas along the airway tree. The other pressures describe different interfaces: transrespiratory pressure is the difference between the airway opening and the body surface and reflects the overall effort to move air against both the chest wall and the lungs; transpulmonary pressure is the difference across the lung wall (airway pressure minus pleural pressure) and governs lung expansion itself; transthoracic pressure is the difference across the chest wall. The key idea is that airflow through the conducting airways is produced by the pressure difference across those airways, i.e., transairway pressure.