Which structure collapses within the lung during a pneumothorax?

During a pneumothorax, air enters the pleural space, leading to a loss of negative pressure that is essential for the normal expansion of the lungs during inhalation. As a result, the alveoli, which are the tiny air sacs where gas exchange occurs, can collapse as the surrounding pressure shifts and becomes equalized with atmospheric pressure.

This collapse impairs the lungs' ability to expand fully, leading to reduced oxygenation of the blood and potential respiratory distress. The bronchi, which are larger airways leading to the lungs, remain open and do not collapse in the same manner under a pneumothorax. Capillaries, while present within the lung tissue and playing a vital role in gas exchange, do not collapse as a direct result of pneumothorax. The diaphragm is a muscular structure that aids in breathing, but it also does not collapse; rather, its function may be compromised due to the lung's inability to expand properly. Thus, the alveoli are the structures most directly affected by the changes in pressure that characterize a pneumothorax.