Formation of intrapleural pressure Air in lungs after delivery Because the elastic recoil causes the lungs to try to collapse, a negative force is always needed to the outside of the lungs to keep the lungs expanded. This force is provided by negative pressure in the normal pleural spaceFormation of intrapleural pressure ◼ Air in lungs after delivery ◼ Because the elastic recoil causes the lungs to try to collapse, a negative force is always needed to the outside of the lungs to keep the lungs expanded. This force is provided by negative pressure in the normal pleural space