Final answer:
A pneumothorax occurs when the pleural sac is pierced, leading to a loss of the negative intrapleural pressure necessary to keep the lungs expanded. This condition allows atmospheric pressure to enter the pleural space and can result in lung collapse.
Step-by-step explanation:
A pneumothorax occurs when the pleural sac is pierced, and intrapleural pressure becomes equal to atmospheric pressure. Intrapleural pressure is always lower than atmospheric pressure due to the elasticity of the lungs and opposing forces from the pleural fluid and thoracic wall which help in maintaining a negative pressure. When this pressure balance is disrupted by an injury or medical condition, it can lead to the lungs collapsing, as the equalization of pressure allows air to rush in and occupy the space of the pleural cavity, eliminating the vacuum that holds the lungs open. Factors that create negative intrapleural pressure include the natural elastance of the lungs and surface tension of the alveolar fluid, with the former being offset by the outward pull from the pleural fluid and chest wall elasticity, resulting in a usual intrapleural pressure of approximately -4 mm Hg. The transpulmonary pressure, which is the difference between the intrapleural and intra-alveolar pressures, dictates lung size.