Lung volumes and capacities, explained
Every lung volume and capacity on a spirogram, and why residual volume, FRC and total lung capacity need more than spirometry.
Define the lung volumes and capacities on a spirogram and explain why residual volume and FRC (and so TLC) cannot be measured by spirometry alone.
The short version
- Tidal volume, inspiratory reserve volume and expiratory reserve volume can be seen directly on a spirogram.
- A capacity is a sum of volumes; vital capacity is tidal volume plus both reserve volumes.
- Residual volume never crosses the mouth, so spirometry cannot measure it.
- Functional residual capacity, residual volume and total lung capacity are all invisible to spirometry.
- Measure FRC by helium dilution, nitrogen washout or body plethysmography, then subtract to find RV.
Lung volumes and capacities: why the lungs are never empty
A man breathes out as hard as he possibly can, until nothing more will come out. His vital capacity is about 4.5 L. Yet his lungs are not empty. How much air is still inside, and why can a spirometer never see it?
This lesson defines every lung volume and capacity on a spirogram, and explains why residual volume and functional residual capacity need a different test from spirometry.[1][2]
The spirogram: tidal volume and the two reserve volumes
A spirometer records air moving in and out of the mouth and draws it as a trace over time.[2] Three volumes can be read straight off that trace.[1]
| Volume | What it is | Typical value |
|---|---|---|
| Tidal volume (TV) | Air moved in one quiet breath | about 500 mL |
| Inspiratory reserve volume (IRV) | Extra air breathed in beyond a normal breath | about 3 L |
| Expiratory reserve volume (ERV) | Extra air pushed out beyond a normal breath out | about 1.2 L |
Values are textbook figures for an adult male and vary with sex, height and age.[1]
Capacities are sums of volumes
A capacity is simply two or more volumes added together.[1]
Everything you can move in or out in one maximal effort. It leaves out the residual volume.
Vital capacity is the sum of all the volumes except residual volume.[1]
The catch: residual volume
After the hardest possible breath out, air is still left in the lungs. That is the residual volume (RV), about 1.2 L.[1][2]
It never leaves because the chest wall cannot squeeze any further and the small airways close, trapping air in the lungs. It also helps keep the alveoli from collapsing.[1]
A spirometer only records air that crosses the mouth, and residual volume never crosses the mouth. So spirometry cannot measure it.[2]
Functional residual capacity (FRC) and total lung capacity
After a quiet breath out, the lungs rest at a point where two forces balance: the lung's elastic recoil pulls inward, and the chest wall's springiness pulls outward.[2]
The air left at that point is the functional residual capacity (FRC). It is the expiratory reserve volume plus the residual volume.[2] With this lesson's typical adult male values (ERV 1.2 L + RV 1.2 L) that is about 2.4 L; textbook figures for FRC range from about 2.4 to 3 L.
About 6 L in a typical adult male.
Measuring the hidden numbers: helium, washout, plethysmography
Functional residual capacity is measured with a method other than spirometry. Three are named in this lesson.[3]
- Helium dilution: the patient breathes a gas mix with a known helium concentration in a closed circuit, and the dilution shows the volume of gas in the lungs at the end of a quiet breath out (FRC).[3]
- Nitrogen washout: another way to measure it.[3]
- Body plethysmography: the patient sits in a sealed booth and the pressure changes give the volume.[3]
Once FRC is known, spirometry fills in the rest.
- VC = 0.5 + 3 + 1.2 = 4.7 L (the opening 4.5 L is a rounded example; these typical values add to 4.7 L)
- RV = FRC − ERV = 2.4 − 1.2 = 1.2 L
- TLC = VC + RV = 4.7 + 1.2 = 5.9 L, about 6 L
Practice question
Original · USMLE styleA 26-year-old man is referred for pulmonary function testing before a diving course. Spirometry shows a tidal volume of 0.6 L, an inspiratory reserve volume of 2.8 L and an expiratory reserve volume of 1.0 L. A helium dilution test gives a functional residual capacity of 2.3 L.
Common questions
What is the difference between a lung volume and a lung capacity?
A volume is a single compartment on the spirogram, such as tidal volume. A capacity is two or more volumes added together, such as vital capacity.
Why can't spirometry measure residual volume?
A spirometer only records air that crosses the mouth, and residual volume never crosses the mouth. It stays in the lungs even after the hardest possible breath out.
How is functional residual capacity measured?
By helium dilution, nitrogen washout or body plethysmography. Residual volume then equals FRC minus expiratory reserve volume.
How do you calculate total lung capacity?
Total lung capacity equals vital capacity plus residual volume, about 6 L in a typical adult male.
Transcript
Read the full narration, by chapter697 words
0:00A breath out, and the lungs are still not empty
A man breathes out as hard as he possibly can, until nothing more will come out. His vital capacity is four and a half litres. Yet his lungs are not empty. How much is still inside, and why can the spirometer never see it? Today: define every lung volume and capacity on a spirogram, and explain why residual volume and functional residual capacity need a different test.
0:25The spirometer, tidal volume and the reserve volumes
Start with the machine. A spirometer records air moving in and out of the mouth, and draws it as a trace over time. One quiet breath is the tidal volume, about 500 mL. Now breathe in as far as you can. The extra air beyond a normal breath is the inspiratory reserve volume, about three litres. Breathe out normally, then push out as much as possible. The extra air beyond a normal breath out is the expiratory reserve volume, about 1.2 litres. Those are three volumes you can see directly: tidal, inspiratory reserve and expiratory reserve.
1:04Capacities are sums of volumes (IC and VC)
A capacity is simply two or more volumes added together. Tidal plus inspiratory reserve is the inspiratory capacity. Tidal plus both reserves is the vital capacity: everything you can move in or out in one maximal effort.
1:20The catch: residual volume
But there is a fourth volume, and it is the catch. After the hardest possible breath out, air is still left in the lungs. That is the residual volume, about 1.2 litres. It never leaves, because the chest wall cannot squeeze any further and the small airways close, trapping air in the lungs. It also helps keep the alveoli from collapsing. A spirometer only records air that crosses the mouth, and residual volume never crosses the mouth. So spirometry cannot measure it.
1:55Functional residual capacity (FRC)
Now the second hidden number. After a quiet breath out, the lungs rest at a point where two forces balance. The lung's elastic recoil pulls inward, and the chest wall's springiness pulls outward. The air left at that point is the functional residual capacity, about 2.4 litres, from the expiratory reserve and residual volumes together. It equals the expiratory reserve volume plus the residual volume. It contains residual volume, so spirometry cannot measure it either.
2:29What spirometry cannot see, and total lung capacity (TLC)
Anything that includes residual volume is invisible to spirometry: residual volume, functional residual capacity and total lung capacity. Total lung capacity is vital capacity plus residual volume, about six litres.
2:45Measuring FRC and RV: helium, washout, plethysmography
So how do we measure them? We measure functional residual capacity with a different method. One is helium dilution: the patient breathes a gas mix with a known helium concentration in a closed circuit, and the dilution shows the volume of the lungs. Another is nitrogen washout. The third is body plethysmography, where the patient sits in a sealed booth and the pressure changes give the volume. Residual volume equals functional residual capacity minus expiratory reserve volume. Total lung capacity equals residual volume plus vital capacity. Spirometry gives the volumes you can move. A second method gives the volume you cannot.
3:29Practice question
Your turn. A 26-year-old man is referred for lung function testing. Spirometry shows a tidal volume of 0.6 litres, an inspiratory reserve volume of 2.8, and an expiratory reserve volume of 1.0. Helium dilution gives a functional residual capacity of 2.3 litres. Which of the following is his total lung capacity? A, 2.3 litres. B, 3.4. C, 4.4. D, 5.7. E, 6.7. Pause here and decide.
4:10The options explained, and the answer
A is out: that is just his functional residual capacity, which is not the total. B is out: that is tidal plus inspiratory reserve, the inspiratory capacity. C is out: that is his vital capacity, and it leaves out the residual volume. E is out: that adds functional residual capacity to vital capacity, which counts the expiratory reserve volume twice. So the answer is D. Residual volume is 2.3 minus 1.0, 1.3; vital capacity 4.4 plus 1.3 is 5.7 litres.
4:51Recap and next lesson
Three things to keep. One: vital capacity is tidal plus both reserve volumes, and capacities are sums of volumes. Two: residual volume, functional residual capacity and anything containing them, like total lung capacity, cannot be measured by spirometry. Three: measure functional residual capacity by helium dilution, washout or plethysmography, then subtract. Next, we use these volumes to tell obstructive from restrictive lung disease.
Sources
Numbers in the text, e.g. [1], point here- OpenStax. Anatomy and Physiology 2e, 22.3 The Process of Breathing (lung volumes and capacities, including inspiratory capacity and TLC). openstax.org/books/anatomy-and-physiology-2e/pages/22-3-the-process-of-breathing
- StatPearls. Physiology, Functional Residual Capacity (abstract, Europe PMC). europepmc.org/article/MED/29763183
- Europe PMC abstracts used for the measurement methods: (residual volume and total lung capacity by body plethysmography and helium dilution) · (multiple-breath nitrogen washout) · (whole-body plethysmography for residual volume) europepmc.org/article/MED/42816439 · europepmc.org/article/MED/42669039 · europepmc.org/article/MED/39308278
The practice question is original and its patient is synthetic. Every fact is checked against the sources above in independent accuracy reviews. Always follow your local guidelines and your own laboratory’s ranges.
Image credits. The spirogram, brackets, equipment sketches, formulas and all handwriting are drawn in code (original). No licensed picture art is used in this lesson. Next: EP 02 · Obstructive vs Restrictive: FEV1/FVC and Flow-Volume Loops
Corrections. None so far. Spotted an error? Email info@medservatory.com with the lesson code MS-S1-RESP-001.
For exam study only, not medical advice. Read the medical disclaimer.
In this series
Playlist on YouTube- EP 01Lung volumes and capacities, explained5:26
- EP 02Obstructive vs restrictive lung disease: FEV1/FVC and loops5:23
- EP 03Lung compliance, Laplace and surfactant: why alveoli stay open5:23
- EP 04V/Q mismatch: shunt, dead space and the oxygen test5:18
- EP 05A-a gradient: the alveolar gas equation and the causes of hypoxaemia5:39
- EP 06Oxygen-hemoglobin dissociation curve: P50, shifts and content5:19
- EP 07CO₂ transport: bicarbonate, the chloride shift, Bohr vs Haldane5:29
- EP 08Central vs peripheral chemoreceptors and oxygen-induced hypercapnia5:27
- EP 09Hypoxic pulmonary vasoconstriction and cor pulmonale5:41
- EP 10CO, cyanide and methaemoglobin: why SpO₂ and PaO₂ fool you5:34