What describes the fluid pressure in the pleural space?

• A. Alveolar pressure
• B. Atmospheric pressure
• C. Intrapleural pressure
• D. Interstitial fluid pressure

Answer: (C)

The fluid pressure in the pleural space is best described by intrapleural pressure. Intrapleural pressure refers to the pressure within the pleural cavity, which is the space between the parietal and visceral pleura surrounding the lungs. This pressure is typically negative relative to both atmospheric pressure and alveolar pressure during normal breathing. This negative pressure is crucial because it helps keep the lungs inflated and facilitates the expansion of the lungs during inhalation.

Intrapleural pressure fluctuates during the respiratory cycle—decreasing during inhalation as the chest expands and increasing slightly during exhalation as the chest cavity contracts. This pressure difference is what aids in the mechanism of ventilation, ensuring that air flows into the lungs.

In contrast, alveolar pressure refers to the pressure inside the alveoli, atmospheric pressure is the pressure exerted by the weight of the atmosphere, and interstitial fluid pressure pertains to the pressure within the spaces between cells in tissues. Each of these pressures plays a role in respiratory physiology, but they do not describe the specific conditions within the pleural space as intrapleural pressure does.