The pleural cavity is a closed cavity enclosed by the dirty pleura and the wall pleura. when breathing the thorax expands, there will be space between the dirty pleura and the wall pleura, forming a negative pressure, generally the size of the negative pressure is 4-6cmH2O. the significance of the negative pressure of the pleural cavity is that it can keep the lungs expanded, when breathing, the negative pressure of the thorax can make the lungs expand quickly. When exhaling, the thorax shrinks and expels the gas from the lungs. Through such negative pressure effect, it helps to keep the respiratory function of the lungs normal.