What is the significance of the pleural cavity's negative pressure?

• A. It keeps the lungs expanded by creating transpulmonary pressure.
• B. It causes alveolar collapse.
• C. It prevents airflow into lungs.
• D. It decreases venous return.

Answer: (A)

The negative pressure in the pleural cavity maintains lung expansion by creating transpulmonary pressure. The pleural space is a small, fluid-filled gap between the lung (visceral pleura) and chest wall (parietal pleura). Because the chest wall tends to expand outward while the lungs tend to recoil inward, a suction develops, keeping the intrapleural pressure below atmospheric. This negative pressure makes the transpulmonary pressure (the difference between airway/alveolar pressure and intrapleural pressure) positive, so the lungs stay inflated and cling to the chest wall. During inspiration, the chest wall moves outward, making intrapleural pressure more negative, which increases transpulmonary pressure and pulls air into the lungs. During quiet expiration, intrapleural pressure becomes less negative but remains below alveolar pressure, allowing the lungs to recoil and air to exit. If this negative intrapleural pressure were lost (for example, air enters the pleural space), the lungs could collapse. So the primary significance is keeping the lungs expanded by maintaining the transpulmonary pressure gradient.