During inspiration, how does alveolar pressure compare to atmospheric pressure?

• A. Alveolar pressure is equal to atmospheric pressure
• B. Alveolar pressure is higher than atmospheric pressure
• C. Alveolar pressure is lower than atmospheric pressure
• D. There is no change in alveolar pressure

Answer: (C)

During inspiration, the diaphragm and intercostal muscles contract, expanding the thoracic cavity. This expansion decreases the pressure within the alveoli, making the alveolar pressure lower than the atmospheric pressure. The pressure gradient created by this difference allows air to flow into the lungs from the atmosphere, as air moves from areas of higher pressure to areas of lower pressure. The decrease in alveolar pressure relative to atmospheric pressure is a fundamental part of the mechanics of breathing, facilitating the intake of oxygen into the lungs.

In contrast, during expiration, the pressure inside the alveoli increases, becoming higher than atmospheric pressure, which is why air is expelled. The equilibrium of alveolar pressure and atmospheric pressure typically occurs at the transitional points of the respiratory cycle, but during the active process of inspiration, it is critical that alveolar pressure remains lower than atmospheric pressure to ensure airflow into the lungs.