What maintains the visceral pleura in contact with the parietal pleura?

• A. Positive pressure between the membranes
• B. Negative pressure (vacuum) between the membranes
• C. Gravity
• D. Air pressure in alveoli

Answer: (B)

The membranes stay in contact because the space between them is kept under negative pressure. This intrapleural vacuum arises from the opposing elastic recoils of the lungs and the chest wall, aided by the surface tension of the small amount of pleural fluid between them. That negative pressure pulls the visceral and parietal pleura together and keeps the lungs expanded within the chest.

At rest this pressure is below atmospheric and becomes even more negative during inspiration, helping the lungs to expand. If air or fluid enters the pleural space and the negative pressure is lost, the lung collapses or fails to fully inflate.

Positive pressure between the membranes would push them apart, gravity isn’t what maintains the contact, and the air pressure in the alveoli isn’t the mechanism for keeping the pleurae apposed.