Final answer:
In pneumothorax, the lung collapses when intrapulmonary pressure is higher than atmospheric pressure due to disruption of the negative intrapleural pressure, thereby causing failure of normal lung expansion.
Step-by-step explanation:
In pneumothorax, the lung collapses because intrapulmonary pressure is higher than atmospheric pressure. Normally, the intrapleural pressure is negative compared to intrapulmonary pressure, ensuring that the lungs remain expanded. When air enters the pleural space, due to a trauma or a spontaneous rupture, the intrapleural pressure can become less negative or positive, disrupting the pressure gradient between the lung and pleural space. The loss of this gradient results in the collapse of the lung because the elastic recoil of lung tissue causes it to shrink to a smaller size.
Transpulmonary pressure, which is the difference between intrapleural and intra-alveolar pressures, normally keeps the lungs open. Intrapulmonary pressure, which should be lower than atmospheric pressure during the process of inhalation, allows air to diffuse into the lungs due to the negative pressure created--this is essential for effective ventilation. If the intrapleural pressure becomes positive compared to the intrapulmonary pressure, as seen in pneumothorax, this mechanism fails, and the lung will not remain inflated.