Functional residual capacity (FRC) is the volume of air that remains in the lungs after normal expiration. FRC can be affected by conditions such as emphysema and obesity. It is measured using gas dilution techniques and whole-body plethysmography and is used to evaluate lung function.
Functional residual capacity (FRC) is the volume of air that remains within the lungs after normal expiration or expiration has occurred. FRC represents the volume of air within the smaller airways and alveoli, or air sacs, that make up the lungs. In conditions such as emphysema, where the lungs become less elastic, there may be a higher than normal FRC. Factors such as obesity can lead to reduced functional residual capacity. FRC is used along with other measurements, known as pulmonary function tests, to evaluate the condition of the lungs.
When a person exhales normally, a volume of air remains in the lungs. This air consists of the expiratory reserve volume, or ERV, and the residual volume, or RV. If, after normal exhalation, a person continues to exhale with as much effort as possible, the extra amount of air exhaled is the ERV. 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, ERV and RV make up the functional residual capacity.
Functional residual capacity measurement can be useful because, when combined with other lung measurements, it can provide a value for total lung capacity, or TLC, which represents the total volume of air the lungs hold when full. As people get older, while total lung capacity remains the same, it is normal for capacity to increase. This is due to a natural loss of spring back in the lungs, which occurs as part of the aging process.
An increase in functional residual capacity can be caused by any condition that leads to overinflation of the lungs. Such conditions can include emphysema and asthma. A decrease in 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 several methods to measure functional residual capacity, including so-called gas dilution techniques and whole body plethysmography. Gas dilution can involve breathing a known amount of helium into a sealed system until a state of equilibrium is reached where the helium concentration in the lungs and the rest of the system is the same. The amount by which the helium has been diluted can be used in the calculation of the functional residual capacity volume. In whole-body plethysmography, a person performs a breathing exercise inside a sealed box. Pressure changes within the box can be measured and used to calculate the volume of air in the lungs which represents the FRC.
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