Between the two layers is the intrapleural space, which normally contains fluid secreted by the membranes. This allows the two layers to slide easily over one another as the lungs inflate and deflate during respiration.
Because it contains liquid and not gas, the space between the pleural layers is sometimes called a ‘potential’ space . It is difficult to compress or expand liquids so the two layers of pleura normally remain tightly adherent to one another.
This enables the lungs, which do not contain any skeletal muscle,