What is the function of pleural fluid?

• A. Keeps the pleural surfaces in contact while allowing them to slide smoothly over one another during breathing
• B. Provides a barrier to infections
• C. Directly expands the lungs
• D. Stores energy for ventilation

Answer: (A)

Pleural fluid mainly serves as a lubricant between the two pleural surfaces—the lung’s visceral pleura and the chest wall’s parietal pleura. This thin film allows them to glide smoothly against each other as the lungs expand and contract, reducing friction and wear during respiration. The fluid also helps create a small amount of surface tension in the pleural space, which, together with the negative intrapleural pressure, helps hold the lungs against the chest wall so their expansion is efficient.

It doesn’t function as a barrier to infection, nor does it directly enlarge the lungs or store energy for ventilation. Lung expansion results from the transpulmonary pressure difference and chest wall mechanics, while energy for breathing is stored in the elastic recoil of lung and chest tissues.