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How does pleural fluid help the lungs move during breathing?

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Illustration showing different animals and their respiratory organs: a spider with book lungs, a chicken with lungs, a sponge with labeled inflow and outflow pores, a fish with gills, and a caterpillar with spiracles. — Respiratory system Different methods of respiration in animals.

The pleura is a serous membrane consisting of two layers: the parietal pleura, which lines the thoracic cavity, and the visceral pleura, which covers the lungs.12

Between these two membranes lies a thin film of extracellular fluid. This fluid serves as a lubricant, enabling the lungs to move smoothly along the walls of the thoracic cavity during breathing. In addition to providing lubrication, the pleural space maintains a negative (below-atmospheric) pressure that keeps the lungs in close apposition to the thoracic wall, ensuring they remain distended against their natural tendency to collapse due to elastic recoil.23

When this system is compromised—such as through inflammation known as pleurisy—respiratory movements can become painful. If the space is breached, such as by air entering the cavity (pneumothorax), the lungs lose this support and will collapse.23