When air or gas builds up in the pleural space (the pleura is a large, thin sheet of tissue that wraps around the outside of your lungs and lines the inside of your chest cavity), this is called a pneumothorax. It is often called a "collapsed lung." It can result from acute lung injury or a lung disease like emphysema. Lung procedures, like surgery, drainage of fluid with a needle, examination of the lung from the inside with a light and a camera, or mechanical ventilation, also can cause it. The most common symptom is sudden pain in one side of the lung and shortness of breath. A pneumothorax also can put pressure on the lung and cause it to collapse. If the pneumothorax is small, it may go away on its own. If it's large, you may need to have a tube placed through your skin and chest wall into the pleural space to remove the air. What Is Haemothorax? Blood also can collect in the pleural space. This is called haemothorax. The most common cause is injury to your chest from blunt force or chest or heart surgery. Haemothorax also can occur in people with lung or pleural cancer. Haemothorax can put pressure on the lung and force it to collapse. It also can cause shock, a state in which not enough blood and oxygen reach important organs in the body. http://www.medic8.com/lung-disorders/pneumothorax.htm