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An open pneumothorax occurs when there is a breach in the chest wall that allows air to enter the pleural space, which should normally be a sealed environment. When this occurs, the pleural space becomes exposed to atmospheric pressure because the opening allows outside air to flow directly into the thoracic cavity.

This exposure can lead to a number of serious complications, including collapse of the lung on the affected side, as the intrapleural pressure, which is typically negative, becomes equalized with atmospheric pressure. During respiration, instead of aiding in lung expansion, the air drawn into the pleural cavity can prevent effective ventilation, leading to respiratory distress and potentially life-threatening situations.

In contrast, the other options do not correctly describe what happens in the case of an open pneumothorax. Fluid pressure refers to the pressure exerted by fluids within vessels and is not applicable here, while positive pressure relates to situations where the pleural area is under pressure greater than atmospheric pressure, such as in tension pneumothorax or mechanical ventilation. Intrathoracic pressure is typically negative in a healthy individual and does not apply to the exposure of the pleural space in an open pneumothorax.