High Quality Content by WIKIPEDIA articles! 1. Pneumothorax: a collection of air within the pleural cavity, arising either from the outside or from the lung. Pneumothoraces may be traumatic, iatrogenic, or spontaneous. A tension pneumothorax is a particular type of pneumothorax where the air may enter (though a defect of the chest wall, lung, or airways) on inspiration, but cannot exit on expiration. Each breath increases the amount of trapped air in the chest cavity, leading to further lung compression. This is a medical emergency. 2. Pleural effusion: a fluid accumulation within the pleural space. Abnormal collections of pleural fluid may be due to excessive fluid volume (i.e. excess intravenous fluids, renal failure), decreased fluid protein (e.g. cirrhosis, proteinuria), heart failure, bleeding (hemothorax), infections (parapneumonic effusions, pleural empyema), inflammation, malignancies, or perforation of thoracic organs (i.e. chylothorax, esophageal rupture).