Pleural fluid primarily serves what function during breathing?

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Pleural fluid's main job is to lubricate the pleural surfaces so the lungs can slide smoothly against the chest wall during breathing. This lubrication minimizes friction as the lungs expand and recoil, and the negative pressure in the pleural space helps keep the lungs pressed against the thoracic wall, enabling easy and efficient expansion.

Surfactant, which is produced by type II alveolar cells, reduces surface tension inside the alveoli to keep them from collapsing between breaths, a function separate from the pleural space. Gas exchange happens across the alveolar–capillary membrane, not via pleural fluid. Osmotic pressure influences fluid balance between compartments but does not drive lung volume increase; the expansion of the lungs is driven by the transpulmonary pressure gradient created by the negative intrapleural pressure.