In which state are the lungs at their functional residual capacity (FRC)?

Functional residual capacity (FRC) refers to the volume of air remaining in the lungs after a normal, passive exhalation. At this point, the lungs are in a state where the forces of the chest wall and the lungs are balanced, allowing for a stable baseline lung volume.

After normal breathing, the body reaches FRC naturally, as this volume represents the equilibrium point where no additional air is being inhaled or exhaled. This state is essential for maintaining optimal gas exchange in the lungs between breaths.

In contrast, after a deep inhalation, the lungs would be at a higher volume than FRC, and after maximum exhalation, they would be below FRC. During rapid breathing, the lungs are in the process of exchanging air more vigorously and are not at FRC, as they are continually moving towards inhalation or exhalation. Understanding these concepts is crucial for recognizing how lung volumes and capacities relate to respiratory function under different conditions.