Which pressure is always compared to atmospheric pressure?

• A. Alveolar pressure
• B. Intrapleural pressure
• C. Expiratory pressure
• D. Functional residual capacity

Answer: (A)

Alveolar pressure, also known as pulmonary pressure, is the pressure within the alveoli of the lungs. It is significant in the mechanics of ventilation because it directly influences the movement of air in and out of the lungs during breathing.

When assessing alveolar pressure, it is typically compared to atmospheric pressure to determine the flow of air. If alveolar pressure is lower than atmospheric pressure, air flows into the lungs (inspiration), and if it is higher, air flows out (expiration). This relationship is fundamental to the mechanics of breathing, as the pressure gradients created between the alveoli and the external environment drive ventilation.

In contrast, intrapleural pressure, which is the pressure within the pleural cavity surrounding the lungs, also relates to atmospheric pressure but in a more indirect manner during respiration. Expiratory pressure is a term that might refer to the pressure in the lungs when one exhales but is not specifically compared against atmospheric pressure as a principle. Functional residual capacity is a measurement related to lung volumes and does not denote a specific pressure that is compared directly to atmospheric pressure.

Thus, alveolar pressure is consistently referenced against atmospheric pressure, making it the correct choice for this question.