Final answer:
The residual volume (RV) is the amount of air that remains in the lungs after a passive exhalation.
Step-by-step explanation:
The volume in the lungs after a passive exhalation is called the residual volume (RV). This is the amount of air that remains in the lungs after a maximal exhalation, and it is important for preventing large fluctuations in respiratory gases. The RV is the only lung volume that cannot be measured directly, but it can be calculated rather than measured.