Accumulation of air in the pleural space.
Occur spontaneously, as a complication of preexisting lung disease, trauma, or an iatrogenic cause.
Pressure in the pleaural space is normally negative in relation to the atmosperic pressure.
Negative pressure is vital to the breathing process
When either the visceral or parietal pleura is breached, air enters the pleural space, equalizing this pressure. Lung expansion is impaired, and the natural recoil tendency of the lung causes it to collapse to a greater or lesser extent, depending on the size and rapidity of air accumulation