Final answer:
The pressure in the pleural cavity is normally lower than atmospheric pressure, maintaining roughly -4 mm Hg to assist lung function and remain expanded during respiration. The correct option is A) Atmospheric.
Step-by-step explanation:
The pressure in the pleural cavity is normally lower than atmospheric pressure. This is due to the unique characteristics of the lungs and the thoracic cavity, which create a space that is negative in pressure compared to the surrounding atmosphere.
The intrapleural pressure fluctuates during the breathing cycle but maintains a general value of approximately -4 mm Hg, suggesting a continuous negative pressure relative to both the intra-alveolar and atmospheric pressures. This negative pressure is crucial for the lungs to function properly as it helps keep them expanded during the breathing process.