Functional Residual Capacity (FRC) Overview Vector

Functional Residual Capacity (FRC): An Overview (2024)

by | Updated: May 30, 2024

Functional residual capacity (FRC) is a crucial physiological parameter that plays a pivotal role in understanding respiratory function and lung health.

It represents the volume of air left in the lungs at the end of a passive exhalation, providing valuable insights into lung mechanics and the body’s ability to maintain efficient gas exchange.

This article breaks down the significance of FRC, its measurement, and its clinical relevance, shedding light on its critical role in assessing pulmonary health.

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What is Functional Residual Capacity?

Functional residual capacity (FRC) is the volume of air remaining in the lungs after a normal, passive exhalation. It’s a critical measurement in respiratory physiology, combining expiratory reserve volume (ERV) and residual volume (RV). FRC reflects lung compliance and is vital for maintaining adequate gas exchange in the lungs.

Functional Residual Capacity (FRC) Lung Box Vector


Functional residual capacity (FRC) is a key lung volume in respiratory physiology, calculated as the sum of expiratory reserve volume (ERV) and residual volume (RV).

Mathmatically, FRC is represented as:



  • Expiratory Reserve Volume (ERV) is the additional air that can be forcibly exhaled after the end of a normal, quiet exhalation. ERV represents the reserve air that can still be expelled from the lungs beyond the regular, passive breath out.
  • Residual Volume (RV) represents the air that remains in the lungs after a maximal exhalation. RV cannot be measured directly through spirometry, as it is the air that keeps the lungs partially inflated at all times to prevent lung collapse.

FRC, therefore, represents the total volume of air remaining in the lungs after a normal exhalation, combining the reserve air (ERV) and the permanently trapped air (RV).

This measurement is crucial in understanding lung function, especially in patients with respiratory diseases, as it provides insight into the lung’s ability to maintain adequate gas exchange and its overall compliance.

Clinical Significance of Functional Residual Capacity

The clinical significance of FRC lies in its role as an indicator of lung health and function. Some key aspects include:

  • Lung Diseases Monitoring: FRC is often altered in lung diseases. In conditions like COPD (Chronic Obstructive Pulmonary Disease) and asthma, FRC is usually increased due to air trapping. Conversely, in restrictive lung diseases like fibrosis, FRC is decreased due to reduced lung compliance.
  • Anesthesia and Surgery: During general anesthesia and surgery, FRC decreases, especially in supine position. This reduction can lead to atelectasis (collapse of lung tissue), impairing oxygenation. Monitoring FRC helps in managing ventilation settings to optimize oxygen delivery and prevent complications.
  • Critical Care: In critically ill patients, especially those on mechanical ventilation, FRC measurement can guide ventilator settings. It aids in the assessment of lung recruitment (reopening collapsed lung areas) and the need for PEEP (Positive End-Expiratory Pressure) to keep the lungs open.
  • Pulmonary Rehabilitation: FRC measurements can guide the effectiveness of pulmonary rehabilitation techniques in chronic lung conditions, helping tailor therapies to improve lung function and overall quality of life.

Note: Understanding FRC helps in diagnosing, monitoring, and managing various pulmonary conditions, making it a valuable tool in respiratory medicine.

How to Measure Functional Residual Capacity

FRC can be measured using several methods, each with its own clinical context and application:

  • Body Plethysmography: Often considered the gold standard, this method involves measuring lung volume in a sealed chamber. The patient breathes against a closed shutter, causing pressure changes that help calculate FRC. It’s accurate but requires specialized equipment.
  • Nitrogen Washout: In this technique, the patient breathes 100% oxygen, and the exhaled nitrogen is measured. The nitrogen concentration decrease is used to calculate FRC. This method is less accurate in patients with severe airway obstruction due to uneven gas mixing.
  • Helium Dilution: Similar to nitrogen washout, this method uses helium instead. The patient breathes a known concentration of helium, and its dilution by the air in the lungs is measured to estimate FRC. It’s limited by the same mixing issues as the nitrogen washout technique.
  • Radiographic Techniques: Chest X-rays and CT scans can be used to estimate lung volumes, including FRC, though they provide less direct measurements and involve radiation exposure.

Note: Each method has its advantages and limitations, and the choice depends on the clinical scenario, patient’s condition, and available resources. Accurate FRC measurement is crucial for diagnosing and managing various respiratory conditions.

FAQs About Functional Residual Capacity

Why is Functional Residual Capacity Important?

Functional residual capacity (FRC) is crucial in respiratory physiology as it indicates the amount of air remaining in the lungs after a normal exhalation.

This residual air ensures continuous gas exchange between breaths, maintains lung compliance, and prevents alveolar collapse.

FRC is particularly important in assessing lung function in various pulmonary diseases, anesthesia, and critical care, as alterations in FRC can indicate underlying respiratory issues or guide ventilation management.

What is the Normal Range for FRC?

The normal range for FRC varies depending on factors like age, gender, body size, and posture. In general, for a healthy adult, FRC is typically about 2.4 to 2.8 liters.

It’s important to note that normal values can differ between individuals and are usually adjusted for height and sex.

Pulmonary function tests are used to measure FRC and compare it against predicted normal values based on these factors.

What is the Difference Between FRC and RV?

The difference between functional residual capacity (FRC) and residual volume (RV) lies in their definitions and roles in lung volumes.

FRC is the total volume of air in the lungs after a normal exhalation, combining the expiratory reserve volume (ERV) and RV.

RV, on the other hand, is the volume of air remaining in the lungs after maximal exhalation, representing the air that cannot be voluntarily expelled.

Thus, RV is a component of FRC, and FRC is always larger than RV as it includes RV plus the ERV.

What is the Difference Between FRC and ERV?

The difference between functional residual capacity (FRC) and expiratory reserve volume (ERV) lies in their definitions and roles in lung physiology.

FRC is the volume of air remaining in the lungs after a normal, passive exhalation and includes ERV plus residual volume (RV).

ERV, in contrast, is the extra volume of air that can be forcibly exhaled after the end of a normal exhalation. While ERV is a component of FRC, FRC is a larger value since it encompasses both ERV and RV.

What is the Difference Between FVC and FRC in the Lungs?

Forced vital capacity (FVC) and functional residual capacity (FRC) are distinct measurements in pulmonary function.

FVC refers to the total volume of air that can be forcibly exhaled after a deep inhalation.

It is a measure of lung capacity and includes expiratory reserve volume (ERV), tidal volume (TV), and inspiratory reserve volume (IRV).

In contrast, FRC is the volume of air remaining in the lungs after a normal, passive exhalation, comprising ERV and residual volume (RV).

Essentially, FVC measures the maximum exhalable air after a full inhalation, while FRC indicates the residual air post-normal exhalation.

Final Thoughts

Functional residual capacity (FRC) is a fundamental concept in the field of respiratory physiology and medicine.

Its measurement and interpretation provide valuable diagnostic information about lung function, aiding in the diagnosis and management of various respiratory conditions.

By understanding FRC, respiratory therapists and healthcare professionals can better assess and address issues related to lung health, ultimately improving patient care and outcomes.

John Landry, BS, RRT

Written by:

John Landry, BS, RRT

John Landry is a registered respiratory therapist from Memphis, TN, and has a bachelor's degree in kinesiology. He enjoys using evidence-based research to help others breathe easier and live a healthier life.


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  • Faarc, Kacmarek Robert PhD Rrt, et al. Egan’s Fundamentals of Respiratory Care. 12th ed., Mosby, 2020.
  • Hopkins E, Sharma S. Physiology, Functional Residual Capacity. [Updated 2022 Dec 26]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2023.
  • OpenStax College, CC BY 3.0., via Wikimedia Commons.

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