What is the FRC?
  • The volume of gas present in the lung at end expiration during tidal breathing
  • Composed of ERV and RV
  • This is usually 30-35 ml/kg, or 2100-2400ml in a normal sized person
  • It represents the point where elastic recoil force of the lung is in equilibrium with the elastic recoil of the chest wall, i.e. where the alveolar pressure equilibrates with atmospheric pressure. 
  • The measurement of FRC is an important starting point for the measurement of other lung volumes
How do you measure the FRC?

Nitrogen dilution method:

  • The subject, whose lungs ought to be full of nitrogen-rich air, is commenced on 100% FiO2.
  • The subject therefore begins to exhale nitrogen
  • With every breath the nitrogen content in their exhaled gas mixture decreases
  • This nitrogen content is measured, as are the exhaled volumes.
  • As all the gas in their chest ends up being replaced by oxygen, the expired nitrogen concentration ends up at zero. Thus, this continues for some period of time, which is long enought to ensure the complete washout of all nitrogen in the chest, and is classically about seven minutes (i.e. 70-80 tidal volumes)
  • Over the course of these seven minutes, all of the nitrogen concentrations and tidal volumes are measured for each breath. Under ideal circumstances, this ends up looking like an exponential curve:
    nitrogen concentration in multiple breath washout method
    Because of this, it is possible to calculate the total exhaled volume of nitrogen.
  • From this total content of nitrogen and the first measured breath's nitrogen concentration it is possible to determine the total lung volume. For example, if the initial N2 concentration was 79% and the final total nitrogen volume was 4L, the total lung volume would have to have been around 5L.

Tracer gas dilution method:

tracer gas dilution method of FRC measurement

In short, the process is as follows:

  • You have a patient with an unknown volume of lung  in which there is a known (zero) concentration of tracer gas.
  • You then give the subject a bolus of tracer gas, with a known volume which contains a known concentration (say, 100%) of the tracer gas. The subject inhales this volume, and it is distributed widely and evenly in their lung volume.
  • The subject then exhales of their intrathoracic gas.
  • The concentration of the tracer in their exhaled gas is measured
  • From this concentration, the volume of distribution of that gas can be calculated, which represents the intrathoracic as volume.


  • The subject and the equipment are all confined in a rigid box which contains a known gas volume.Measurement of FRC by plethysmography
  • The measurement equipment is totally enclosed and so the total amount of gas in there is constant.
  • The pressure in the patient's lung and in the box are measured. 
  • Now, let's say the the subject exhales against a closed airway.
    Measurement of FRC by plethysmography
  • As the subject exhales,
    • Intrathoracic volume decreases, which means the volume of the box increases (as the walls are rigid and there is a finite volume shared by the chest and the box).
    • Intrathoracic pressure increases, and therefore box pressure decreases proportionally.
  • Though the amount of the gas in the chest is unknown, we know that (according to Boyle's law) the product of pressure and volume in the chest should be the same as the product of volume and pressure in the box.
  • The volume in the box, the pressure in the box and the pressure in the chest are all known variables at this point, leaving the volume of intrathoracic gas as the last unknown
What factors influence the FRC?
Factors which Influence the Functional Residual Capacity
Factors which increase FRC Factors which decrease FRC
Factors which influence lung size
Increased height Short stature
Male gender Female gender
Age: ratio of FRC to total lung capacity increases, but absolute FRC remains stable 
(Wahba et al, 1983)
Factors which influence lung compliance
Increased compliance due to disease, eg. emphysema Decreased lung compliance due to disease, eg. ARDS
Increased end-expiratory pressure, eg. PEEP or auto-PEEP D
Factors which influence chest expansion and chest wall compliance
Open chest or mediastinum Increased intraabdominal pressure:
pregnancy, ascites, abdominal surgery
  Decreased respiratory muscle tone, eg. anaesthesia/sedation
Upright position and prone position Supine and head down position
  Circumferential burns, chest binder devices (eg. post mastectomy)
Why is this gas volume important?
  • It keeps small airways open. At FRC, the small airways are kept splinted open by the tension of the surrounding lung tissue. If the FRC is reduced below the closing capacity, there will be gas trapping and atelectasis. 
  • It is representative of compliance. Any decrease in lung compliance (i.e. due to decreased chest wall compliance or due to decreased lung tissue compliance) causes a decrease in FRC.
  • It represents optimal compliance. At FRC, the pressure-volume curve which represents compliance is at its steepest, which means the work of breathing required to inflate the lung from FRC is at its minimum. 
  • It keeps a gas reserve between breaths.
  • It keeps pulmonary vascular resistance at a minimum. FRC is where pulmonary vascular resistance is at its lowest, representing the bottom of the U-shaped PVR-volume curve.
  • Relationship between FRC and closing capacity influences the development of atelectasis and shunt 
What are the consequences of a decreased FRC?
  • Decreased oxygen reserves: because the FRC acts as the main oxygen reservoir, the loss of volume here will give rise to an increased fluctuation in the bloodstream oxygen contentbetween breaths, and during episodes of apnoea.
  • Increased atelectasis: Decreasing the FRC to below the closing capacity tends to produce resorption atelectasis, as small airways close in expiration.
  • Increased shunt:  The consequence of abovementioned atelectasis will be shunt, i.e regions of lung which do not participate in gas exchange because they are not ventilated.
  • Decreased lung compliance: the decreasing size of alveoli at lower FRCs results in a decreased rate of 
  • Increased airway resistance: because airway resistance is relatively low at FRC, it is going to increase as the FRC decreases. This is due to the fact that collapsing alveoli tend to stop providing the radial traction which keeps the small airways open.
  • Increased work of breathing, owing to the above.
  • Decreased tidal volume and increased respiratory rate, due to decreased lung compliance 
  • Decreased tolerance of position changes, i.e. with a low baseline FRc in the upriht position a patient will not tolerate being supine for very long, as the FRC will drop yet further 
  • Increased pulmonary vascular resistance, partly due to the effect of narrowed alveoli on perialveolar vessel caliber and partly owing to the inevitable increase in collapsed hypoxic lung regions which promote hypoxic pulmonary vasoconstriction.
  • Increased right ventricular afterload, which is due to the increase in pulmonary pressure


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