At rest, what is the measured difference between alveolar pressure (PA) and barometric pressure (PB)?

• A. 1
• B. 0
• C. -1
• D. 2

Answer: (B)

At rest, the measured difference between alveolar pressure (PA) and barometric pressure (PB) is zero. This situation reflects the mechanics of the respiratory system when it is at equilibrium, meaning that during the resting phase of the respiratory cycle, the pressures within the alveoli are equal to the external atmospheric pressure.

When there is no airflow into or out of the lungs, the alveolar pressure stabilizes, leading to a balance where the pressures equalize. This balance is crucial because it prevents air from entering or leaving the lungs unless there is a change in pressure due to breathing actions, like inhalation or exhalation, which actively adjusts PA relative to PB. Therefore, at rest, the difference is zero, indicating there is no net movement of air.