pleura that covers the lung and the parietal pleura that lines the inside of the chest wall. The space between the two layers is called the intrapleural space. Because the pressure in the pleural cavity is below atmospheric pressure (intrapleural pressure), while the pressure in the lungs is higher, close to atmospheric pressure, the lung must expand as it follows the inspiratory movements of the thorax and the diaphragm. The fluid in the pleural cavity prevents excessive friction between the two pleural layers.