“The pleura has fluid between the pleura parietal and visceral layer membranes of the lungs. The function of the pleural fluid is to decrease friction between the parietal and visceral pleura during respiration, allow apposition of the lungs to the chest wall and ensure coupling between the lung, chest wall and diaphragm to optimize ventilation.
The production and reabsorption of fluid is dictated by the Staling forces of parietal and visceral pleura capillaries (hydrostatic pressure, plasma oncotic pressure), as well as capillary permeability and the negative intrathoracic pressure. The lymphatics also have a significant ability to reabsorb large protein molecules and fluid. Any disruption or alterations of these mechanisms may lead to a pleural effusion.
Pleural fluid moves via a net filtration gradient from the parietal pleura capillaries into the pleura, while fluid is reabsorbed generally by parietal pleural lymphatics.”
National Library of Medicine (Pleural Disorders – PMC)