Pneumothorax refers to gas within the pleural space. Normally, the alveolar pressure is greater than the intrapleural pressure, while the intrapleural pressure is less than atmospheric pressure. Therefore, if a communication develops between an alveolus and the pleural space or between the atmosphere and the pleural space, gases will follow the pressure gradient and flow into the pleural space. This flow will continue until the pressure gradient no longer exists or the abnormal communication has been sealed. Because the thoracic cavity is normally below its resting volume, and the lung is above its resting volume, the thoracic cavity enlarges and the lung becomes smaller when a pneumothorax develops.