What is residual volume?

Residual volume refers to the amount of air that remains in the lungs after a person has exhaled as much as possible. This volume is crucial because it prevents the lungs from collapsing completely and allows for continuous gas exchange to occur, even between breaths. Even during forced expiration, there is always some air left in the lungs, which is measured as residual volume.

Understanding the other options helps to clarify why they do not define residual volume. The volume of air exchanged during normal breathing describes tidal volume, which is distinctly different as it pertains to the amount of air inhaled or exhaled during a typical breath. The volume of air that can be forcibly inhaled refers to inspiratory reserve volume, i.e., the extra air that can be inhaled after a normal breath, which is also not the same as residual volume. Lastly, the total volume of air in the lungs encompasses all lung volumes, including residual volume, but it does not specifically refer to just the air that remains after maximum exhalation. Therefore, identifying the amount of air left after a maximal expiration directly defines residual volume, confirming its significance in respiratory physiology.