What is Functional Residual Capacity?

Functional Residual Capacity (FRC) is the air volume that remains inside the lungs after normal expiration, or breathing out, has occurred. FRC represents the volume of air inside the smaller breathing passages and the alveoli, or air sacs, that make up the lungs. In conditions such as emphysema, where the lungs become less elastic, there may be a greater than normal FRC. Factors such as obesity can lead to a decreased functional residual capacity. FRC is used along with other measurements, known as pulmonary function tests, to assess the condition of the lungs.

When a person breathes out normally, a volume of air remains inside the lungs. This air consists of the expiratory reserve volume, or ERV, and the residual volume, or RV. If, following normal expiration, a person continues to breathe out with as much effort as possible, the extra amount of air that is exhaled is the ERV. The residual volume, or RV, represents the small amount of air that remains inside the alveoli after the ERV has been expelled from the lungs. Together, the ERV and the RV make up the functional residual capacity.

Measuring functional residual capacity can be useful because, when combined with other lung measurements, it can provide a value for the total lung capacity, or TLC, which represents the total volume of air that the lungs contain when full. As people get older, while total lung capacity stays the same, it is normal for the capacity to increase. This is due to a natural loss of elastic recoil in the lungs, occurring as part of the aging process.

An increased functional residual capacity may be caused by any conditions which lead to excessive inflation of the lungs. Such conditions may include emphysema and asthma. Decreased inflation, and a lower FRC, occur when people are lying down or have been sedated and in those who are obese.

Respiratory physiology laboratories may use different methods to measure functional residual capacity, including what are known as gas dilution techniques, and whole body plethysmography. Gas dilution may involve breathing in a known amount of helium in a sealed system, until a state of balance is reached, where the concentration of helium in the lungs and in the rest of the system is the same. The amount by which the helium has been diluted can be used in calculating the volume of the functional residual capacity. In whole body plethysmography, a person carries out a breathing exercise inside a sealed box. Changes in the pressure inside the box can be measured and used to calculate the volume of air in the lungs that represents FRC.