Final answer:
A puncture allowing air into the pleural space is called a pneumothorax, caused by air disrupting the negative pressure necessary for lung expansion, potentially leading to a collapse of the lung.
Step-by-step explanation:
A puncture to the pleural space by a knife, a gun, or a broken rib that allows air between the parietal and visceral pleura is called a pneumothorax. This condition arises when air enters the pleural cavity, the space between the visceral layer covering the lungs and the parietal layer lining the thoracic cavity. This air disrupts the negative pressure ordinarily maintained within the pleural space, which is essential for proper lung expansion. When the pleura is punctured, as might happen with a knife wound, gun, or broken rib, this can cause a collapse of the lung, a situation known as a pneumothorax.
Spontaneous pneumothorax is another type linked to subtle connective tissue abnormalities in the lungs, though in the context of this question, the puncture described is likely to result in a traumatic, not spontaneous, pneumothorax. In cases where air pressure in the pleural space increases to the point where it compresses the lung and shifts the mediastinum to the opposite side, it is specifically termed a tension pneumothorax, which is an emergency situation requiring immediate medical intervention.