Final answer:
Mandatory breaths during PC-CMV are cycled into expiration by time, and Boyle's law explains the pressure-volume relationship in the lungs. Residual volume is necessary to prevent alveolar collapse.
Step-by-step explanation:
In pressure-controlled mechanical ventilation (PC-CMV), mandatory breaths are cycled into expiration by time. This type of ventilation ensures that a preset pressure is maintained in the airways during inspiration and that the breath is terminated after a predefined period. As such, the answer to the student's question is A) Time. It's important to understand that respiratory mechanics follow Boyle's law, which asserts that the pressure of a gas is inversely proportional to its volume. Hence, during normal breathing or mechanical ventilation, an increase in lung volume during inspiration leads to a decrease in pressure, allowing air to flow into the lungs, while a decrease in lung volume during expiration increases the pressure and facilitates the expulsion of air.
To prevent the collapse of alveoli, residual volume is crucial. This is the amount of air that remains in the lungs after maximal exhalation and ensures that the alveoli remain inflated and are not completely deflated, avoiding collapse or atelectasis.