The pleural cavity is located between two membranes (visceral membrane and parietal membrane) that are between the lungs and chest wall. The pleural cavity is filled with pleural fluid, which holds the two membranes together through pressure and tension. The pleural cavity is held in a constant state of negative...
pressure allowing the pressure within the lungs to increase and decrease as we exhale and inhale, respectively.
A patient that has a chest injury that has penetrated the pleural cavity will have a mixture of blood and air enter the pleural cavity. This equalizes the pressure of the pleural cavity with that of the surrounding atmosphere instead of allowing the pleural cavity to remain in a state of negative pressure. This also causes the visceral membrane and parietal membrane to separate. Furthermore, when the rib cage moves out, the lungs do not expand with the rib cage.
Due to the injury, the lungs cannot expand, and the pressure within the lungs drops and remains low. This also means that respiration is suspended, and air cannot be pulled into the bronchi. The lung affected by the injury will shrivel up and become known as a "collapsed" lung. The patient will likely need to undergo surgery to close the penetration and possibly insert a chest drain to drain the blood from the pleural cavity.