There are clinical emergencies when the important negative pressure inside the pleural cavity becomes compromised. If air is introduced into the pleural cavity from a stab wound or a gunshot, the lungs will also collapse. Air may also be introduced internally such as when a rib cuts the surface of the lung or when an alveolus ruptures.

Air in the pleural cavity is called a pneumothorax. A tension pneumothorax occurs when air is allowed to enter the pleural cavity during inspiration but not exit during expiration. A one-way valve forms from damaged tissue. Pressure builds up in the thoracic cavity and may put pressure on veins returning blood to the left side of the heart for a decrease in venous return and the decreased cardiac output may be life-threatening. In an ER environment, a large needle is inserted between the ribs into the pleural cavity to relieve the pressure from a tension pneumothorax. In the movie "Just Like Heaven", a doctor played by Reese Witherspoon saved a man suffering from tension pneumothorax in a restaurant. She made an incision into the thorax with a kitchen knife and then shoved the tube part of a pen between his ribs to relieve the pressure. 

By National Heart Lung and Blood Institute, License: Wikimedia Commons Link: https://commons.wikimedia.org/wiki/File%3APleurisy_and_pneumothorax.jpg

Pleuritis is an infection of the pleura and may cause excess fluid to build up in the pleural cavity. With too much fluid, the visceral and parietal layers are no longer attracted to each other and the lung may collapse.

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