FRC equals CC in which of the following scenarios?

The correct answer highlights the relationship between Functional Residual Capacity (FRC) and Closing Capacity (CC) as individuals age and change positions. FRC refers to the volume of air remaining in the lungs after a normal expiration, while Closing Capacity is the volume in the lungs at which the small airways begin to close during expiration.

As a person ages, several physiological changes occur, including loss of lung elasticity, decreased chest wall compliance, and alterations in the respiratory muscles' strength. This affects how FRC and CC correlate.

By mid 40s, individuals in a supine position may still maintain a normal relationship between FRC and CC. However, as aging progresses into the mid 60s, particularly when upright, there is an increase in the tendency for CC to rise relative to FRC. Therefore, FRC equals CC by the mid 40s when supine and by the mid 60s when upright due to these age-related changes in lung mechanics.

This explanation clarifies why the other scenarios do not accurately depict the correlation between FRC and CC, as they either do not accurately represent the necessary lifespan changes or the specific postural influences. This is crucial for understanding respiratory physiology and potential clinical implications during aging.