Table of ContentsView AllTable of ContentsFRC and Lung FunctionHow It’s MeasuredInterpreting ResultsAdditional Testing
Table of ContentsView All
View All
Table of Contents
FRC and Lung Function
How It’s Measured
Interpreting Results
Additional Testing
Functional residual capacity (FRC) is the volume of air left in your lungs after a normal, passive exhalation. This test is used to evaluate your lung function, and you may need to have your FRC measured before lung surgery or if you have a lung disease such as emphysema orchronic obstructive pulmonary disease (COPD).
Calculation of your FRC is a bit complicated, because this number reflects the air remaining inside your lungs, which cannot be directly measured. Tests that measure FRC include alung plethysmographor a helium dilution method.
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What FRC Says About Lung Function
You regularly breathe in and out at a rate of 10 to 20 breaths per minute. Even though you exhale, air is still left in the lungs. The volume of remaining air after a normal breath is called the FRC.
As you breathe, youralveoli(tiny air sacs) always stay open. This is due to a number of factors. Surfactant, a sticky substance that lines the inside of your alveoli, helps keep them open.
The elasticity of the lungs also keeps your alveoli partially inflated. Because your lungs remain partially open between breaths, it is easier for you to inhale. Less elasticity means more trouble breathing.
FRC is a reflection of how elastic your lungs are, and a medical team may use your FRC measurements to evaluate how well the treatment you are using for your lung disease is working or whether your condition is progressing.
Results can also help in the diagnosis of lung disease—specifically when differentiating betweenrestrictive and obstructive lung disease.
FRC = ERV (the amount of extra air you can exhale if you do so forcefully) + RV (the amount of air remaining in the lungs no matter the extent of exhalation)
How Breathing Works in Your Lungs
How FRC Is Measured
The tests used to measure FRC rely on indirect calculations to determine the volume of air in your lungs after you breathe out. These tests require your cooperation, and you will be asked to follow certain instructions as you inhale and exhale.
Lung Plethysmography
This procedure, also called pulmonary plethysmography or whole-body plethysmography, requires that you enter into a small chamber and breath into a mouthpiece. As you breathe in and out, the air pressure inside the chamber is measured. The calculation of your FRC is based on a mathematical formula that incorporates the amount of air that you breathe in and out, as well as the volume of air in your lungs.
Helium Dilution Method
This test uses aspirometer, which is a simple device that you can breathe into and out of. When you have a helium dilution test, you use a spirometer that contains helium. As you inhale and exhale, the concentration of helium remaining in the spirometer can be used to calculate your FRC.
Tests used to measure your FRC are safe, even if you have advanced lung disease.However, helium dilution can underestimate FRC in people with lung bullae and/or COPD.
Interpreting Your FRC Results
Normally, FRC values range between approximately 1800 ml to 2500 ml. But this number depends on several factors, such as age, weight, height, gender, and pregnancy. Major alterations in your FRC can indicate the presence or progression of lung disease.
When you have your FRC measured, you are likely to have otherpulmonary function tests (PFTs)as well.
An Overview of Pulmonary Function Tests
A Word From Verywell
If you have pulmonary disease, a healthcare provider may prescribe respiratory therapy and physical therapy to help improve your breathing abilities. You can benefit from doing these exercises with a therapist, or you may learn how to do them on your own at home.
5 SourcesVerywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read oureditorial processto learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.Hopkins E, Sharma S.Physiology, Functional Residual Capacity. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing;Selvi E C, K V Rao K, Malathi.Should the Functional Residual Capacity be Ignored?.J Clin Diagn Res. 2013;7(1):43–45. doi:10.7860/JCDR/2012/4876.2666Borg BM, Thompson BR.The Measurement of Lung Volumes Using Body Plethysmography: A Comparison of Methodologies.Respiratory Care. 2012;57(7):1076-1083. doi:10.4187/respcare.01444O’Donnell CR, Bankier AA, Stiebellehner L, Reilly JJ, Brown R, Loring SH.Comparison of plethysmographic and helium dilution lung volumes: which is best for COPD?.Chest. 2010;137(5):1108–1115. doi:10.1378/chest.09-1504Wanger J, Clausen JL, Coates A.Standardisation of the measurement of lung volumes.European Respiratory Journal. 2005;26(3):511-522. doi:10.1183/09031936.05.00035005Additional ReadingDelgado BJ, Bajaj T.Physiology, Lung Capacity. StatPearls Publishing.Muñoz Cofré R, Del Sol M, Medina González P, Escobar Inostroza J, Lizana PA, Conei D, Escobar Cabello M.Relation among body mass index, waist-hip ratio, and pulmonary functional residual capacity in normal weight versus obese Chilean children: A cross-sectional study. Arch Argent Pediatr. 2019 Aug 1;117(4):230-236. doi:10.5546/aap.2019.eng.230.Takeuchi Y, Kato H, Ishizaka M, Kubo A.Effects of increased functional residual capacity on finger-floor distance in healthy young adults. J Phys Ther Sci. 2019 Jan;31(1):29-32. doi:10.1589/jpts.31.29. Epub 2019 Jan 10.
5 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read oureditorial processto learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.Hopkins E, Sharma S.Physiology, Functional Residual Capacity. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing;Selvi E C, K V Rao K, Malathi.Should the Functional Residual Capacity be Ignored?.J Clin Diagn Res. 2013;7(1):43–45. doi:10.7860/JCDR/2012/4876.2666Borg BM, Thompson BR.The Measurement of Lung Volumes Using Body Plethysmography: A Comparison of Methodologies.Respiratory Care. 2012;57(7):1076-1083. doi:10.4187/respcare.01444O’Donnell CR, Bankier AA, Stiebellehner L, Reilly JJ, Brown R, Loring SH.Comparison of plethysmographic and helium dilution lung volumes: which is best for COPD?.Chest. 2010;137(5):1108–1115. doi:10.1378/chest.09-1504Wanger J, Clausen JL, Coates A.Standardisation of the measurement of lung volumes.European Respiratory Journal. 2005;26(3):511-522. doi:10.1183/09031936.05.00035005Additional ReadingDelgado BJ, Bajaj T.Physiology, Lung Capacity. StatPearls Publishing.Muñoz Cofré R, Del Sol M, Medina González P, Escobar Inostroza J, Lizana PA, Conei D, Escobar Cabello M.Relation among body mass index, waist-hip ratio, and pulmonary functional residual capacity in normal weight versus obese Chilean children: A cross-sectional study. Arch Argent Pediatr. 2019 Aug 1;117(4):230-236. doi:10.5546/aap.2019.eng.230.Takeuchi Y, Kato H, Ishizaka M, Kubo A.Effects of increased functional residual capacity on finger-floor distance in healthy young adults. J Phys Ther Sci. 2019 Jan;31(1):29-32. doi:10.1589/jpts.31.29. Epub 2019 Jan 10.
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read oureditorial processto learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
Hopkins E, Sharma S.Physiology, Functional Residual Capacity. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing;Selvi E C, K V Rao K, Malathi.Should the Functional Residual Capacity be Ignored?.J Clin Diagn Res. 2013;7(1):43–45. doi:10.7860/JCDR/2012/4876.2666Borg BM, Thompson BR.The Measurement of Lung Volumes Using Body Plethysmography: A Comparison of Methodologies.Respiratory Care. 2012;57(7):1076-1083. doi:10.4187/respcare.01444O’Donnell CR, Bankier AA, Stiebellehner L, Reilly JJ, Brown R, Loring SH.Comparison of plethysmographic and helium dilution lung volumes: which is best for COPD?.Chest. 2010;137(5):1108–1115. doi:10.1378/chest.09-1504Wanger J, Clausen JL, Coates A.Standardisation of the measurement of lung volumes.European Respiratory Journal. 2005;26(3):511-522. doi:10.1183/09031936.05.00035005
Hopkins E, Sharma S.Physiology, Functional Residual Capacity. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing;
Selvi E C, K V Rao K, Malathi.Should the Functional Residual Capacity be Ignored?.J Clin Diagn Res. 2013;7(1):43–45. doi:10.7860/JCDR/2012/4876.2666
Borg BM, Thompson BR.The Measurement of Lung Volumes Using Body Plethysmography: A Comparison of Methodologies.Respiratory Care. 2012;57(7):1076-1083. doi:10.4187/respcare.01444
O’Donnell CR, Bankier AA, Stiebellehner L, Reilly JJ, Brown R, Loring SH.Comparison of plethysmographic and helium dilution lung volumes: which is best for COPD?.Chest. 2010;137(5):1108–1115. doi:10.1378/chest.09-1504
Wanger J, Clausen JL, Coates A.Standardisation of the measurement of lung volumes.European Respiratory Journal. 2005;26(3):511-522. doi:10.1183/09031936.05.00035005
Delgado BJ, Bajaj T.Physiology, Lung Capacity. StatPearls Publishing.Muñoz Cofré R, Del Sol M, Medina González P, Escobar Inostroza J, Lizana PA, Conei D, Escobar Cabello M.Relation among body mass index, waist-hip ratio, and pulmonary functional residual capacity in normal weight versus obese Chilean children: A cross-sectional study. Arch Argent Pediatr. 2019 Aug 1;117(4):230-236. doi:10.5546/aap.2019.eng.230.Takeuchi Y, Kato H, Ishizaka M, Kubo A.Effects of increased functional residual capacity on finger-floor distance in healthy young adults. J Phys Ther Sci. 2019 Jan;31(1):29-32. doi:10.1589/jpts.31.29. Epub 2019 Jan 10.
Delgado BJ, Bajaj T.Physiology, Lung Capacity. StatPearls Publishing.
Muñoz Cofré R, Del Sol M, Medina González P, Escobar Inostroza J, Lizana PA, Conei D, Escobar Cabello M.Relation among body mass index, waist-hip ratio, and pulmonary functional residual capacity in normal weight versus obese Chilean children: A cross-sectional study. Arch Argent Pediatr. 2019 Aug 1;117(4):230-236. doi:10.5546/aap.2019.eng.230.
Takeuchi Y, Kato H, Ishizaka M, Kubo A.Effects of increased functional residual capacity on finger-floor distance in healthy young adults. J Phys Ther Sci. 2019 Jan;31(1):29-32. doi:10.1589/jpts.31.29. Epub 2019 Jan 10.
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