Functional residual capacity (FRC) is the amount of air that remains in your lungs after a normal and passive exhalation. This air is the only source of oxygen and carbon dioxide exchange between your lungs and blood when you exhale.

Doctors use FRC and a variety of other lung function tests to screen for respiratory diseases. If your FRC is higher or lower than predicted based on factors like your age, height, and sex, it may indicate a restrictive or obstructive respiratory condition.

Read on to learn more about what FRC means, how it’s measured, and how your FRC score is interpreted.

Your lungs are never completely empty, even when you breathe out as much as you can. The amount of air left in your lungs after a normal, passive breath is known as your FRC.

Perhaps, an easy way to understand FRC is by thinking of your lungs as an elastic band.

Breathing processStretching an elastic
You inhale a normal breath. This puts your lungs as well as your joints, muscles, and ribs under stretch.You pull an elastic band apart with your two hands.
When you exhale, the stretch you created in your lungs and other tissues allows your lungs to passively return to their resting size. When you let go of the elastic, it returns to its original length.
The amount of air left in your lungs after exhaling is your FRC. Under conscious control, you can exhale even more air, and in a lab, scientists could extract even more air.Even though the elastic is at its original length, you can shrink it farther by forcefully balling it in your hands.

FRC and spirometry

Spirometry is a test doctors use to measure your lung function. It involves breathing into a machine called a spirometer that measures how much air you breathe in and out.

FRC cannot be directly measured during spirometry because you cannot completely empty your lungs. Even if you breathe out as much as you can, about half of your FRC still remains. This air is known as your residual volume.

Spirometry and helium dilution

Doctors have to calculate FRC indirectly when using spirometry. They typically use a technique called the helium dilution method.

During the helium dilution method, the spirometer is filled with a known concentration of helium, and you inhale and exhale through the machine. Helium is practically insoluble in the blood so over time the amount of helium in the spirometer and your lungs will reach equilibrium, which allows your FRC to be calculated mathematically.

FRC and plethysmography

During plethysmography, you enter an airtight chamber and breathe through a tube. While you’re breathing, a shutter will drop across the breathing tube, and you’ll continue to attempt to breathe. An increase in your chest volume will slightly increase pressure in the chamber, which can be used to measure your FRC.

In most people without lung problems, FRC is about 3 liters (3,000 milliliters).

FRC increases with height and decreases with age and pregnancy. It’s also generally lower in women and people who do not have obesity.

If your FRC is lower or higher than expected based on your demographic, your doctor may suspect an underlying respiratory condition.

Other lung function values are more widely used than FRC for measuring lung function, but it still plays a role in helping doctors differentiate between restrictive and obstructive lung disease, as well as monitor treatment effectiveness.

Restrictive lung diseases often cause a loss in lung elasticity that decreases your total lung capacity. And decreased total lung capacity results in decreased FRC.

Obstructive lung diseases like emphysema often lead to an increased FRC due to trapped air in the lung and increased lung compliance.

Pulmonary function test results can help doctors understand how well your lungs are working. In many cases, your doctor will consider multiple scores when making a diagnosis.

Vital capacity (VC)

VC is the largest volume of air you can exhale after inhaling a full breath. It can be used to help diagnose some lung conditions. VC is generally decreased in restrictive lung disease and about typical in obstructive lung disease.

Residual volume (RV)

RV is the largest volume of air remaining in your lungs after maximum exhalation, as opposed to FRC, which is the amount remaining after a normal breath. RV makes up about half of FRC.

Increased RV can be used to help diagnose some obstructive lung conditions like chronic obstructive pulmonary disease.

Total lung capacity (TLC)

Total lung capacity is the volume of air in your lungs when you inhale maximally. It can be calculated as the sum of your VC and RV. Restrictive lung disease leads to decreased TLC. Trapped air in people with obstructive lung disease causes an increase in TLC.

Forced vital capacity (FVC)

FVC is the maximum amount of air you can exhale after fully inhaling. It makes up about 80 percent of your total lung capacity since some air always remains in your lungs.

FVC is used along with other lung volume measurements to diagnose obstructive lung conditions.

Tidal volume (TV)

TV is the volume of air that moves into or out of your lungs each time you breathe. It’s roughly 500 and 400 milliliters in a healthy male and female, respectively.

Ensuring adequate TV is important for people on mechanical ventilation to make sure they’re receiving adequate air but not so much to damage the lungs.

Here are some questions people often have about FRC.

What does increased FRC mean?

An increased FRC can be a sign of increased lung compliance from an obstructive lung condition like emphysema.

What does decreased FRC mean?

A decreased FRC can be a sign of a restrictive respiratory condition, such as pulmonary fibrosis, that causes decreased compliance in your lungs.

FRC is a measure of the amount of oxygen that remains in your lungs after you passively exhale. Knowing your FRC can help doctors diagnose obstructive or restrictive lung conditions and monitor treatment progress.

A doctor can help you decide if you may benefit from FRC testing or whether other functional lung tests may be more appropriate.