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# Lung volumes, capacities and spirometry

- Measurement
- Exam priority: high

The lung has four basic volumes; their sums are the capacities. Spirometry measures most of them, but not the air that always stays inside.

Updated 2026-11-01

Also called: Spirometry, Pulmonary function tests

## Key points

- Capacities are sums: IC = TV + IRV; FRC = ERV + RV; VC = IRV + TV + ERV; TLC = VC + RV.
- Spirometry does NOT measure RV, FRC or TLC: these need helium dilution, nitrogen washout or body plethysmography.
- FRC is the resting volume of the system: inward lung recoil balances outward chest wall recoil.
- Normal FEV₁/FVC ≳ 0.70 (or above the lower limit of normal for age).
- Values depend on height, age, sex and ethnicity, so they are reported as percent predicted.

## What it is

The lung volumes are tidal, inspiratory reserve, expiratory reserve and residual; a capacity is the sum of two or more. Spirometry records air moving in and out, so it cannot measure residual volume or any capacity that contains it. FEV₁ and FVC from a forced expiration are the basis for sorting obstructive from restrictive patterns.

## Key facts

- Residual volume keeps alveoli from collapsing completely and keeps gas exchange going between breaths.
- FRC is a buffer: it prevents large swings in alveolar PO₂ with each breath.
- FRC falls when supine and in obesity, because the abdominal contents push the diaphragm up.

### Lung volumes and capacities

**TLC** 6.0 L **VC** 4.8 L **FRC** 2.4 L **RV** 1.2 L **FEV1/FVC** 0.80

Normal adult pattern (approximate values: they vary with height, age and sex).

Tap a volume, a capacity or FEV1.

- IRV · Inspiratory reserve volume
- TV · Tidal volume
- ERV · Expiratory reserve volume
- RV · Residual volume
- Capacity (sum of volumes)
- Current forced expiration
- Normal

Illustrative spirogram drawn by Durgical; approximate adult values (OpenStax) and schematic patterns, not for reading a real spirometry.

## Equations

**Total lung capacity**TLC = VC + RV

**VC**
vital capacity

**RV**
residual volume

Because RV cannot be exhaled, TLC needs a gas method or plethysmography.

**Functional residual capacity**FRC = ERV + RV

**ERV**
expiratory reserve volume

**RV**
residual volume

The volume at the end of a quiet expiration.

## Typical values (young adult, ≈ 70 kg; vary with height and sex)

| Parameter | Value | Note |
| --- | --- | --- |
| Tidal volume (TV) | ≈ 500 mL | |
| Inspiratory reserve volume (IRV) | ≈ 3000 mL | |
| Expiratory reserve volume (ERV) | ≈ 1100–1200 mL | |
| Residual volume (RV) | ≈ 1200 mL | not by spirometry |
| Functional residual capacity (FRC) | ≈ 2300–2400 mL | not by spirometry |
| Vital capacity (VC) | ≈ 4600–4800 mL | |
| Total lung capacity (TLC) | ≈ 5800–6000 mL | not by spirometry |
| FEV₁/FVC | ≳ 0.70–0.80 | lower in older adults |

Approximate values for a healthy adult at rest; each laboratory sets its own ranges.

## Clinical notes

- Asthma and COPD: air trapping; RV and FRC rise and VC may fall even with a normal or high TLC.
- Interstitial lung disease: all volumes shrink; a low TLC is what confirms restriction.
- A poorly performed test (weak effort, short blow) underestimates FVC and can mimic restriction.

## Memory aid

If the name contains 'residual' or 'total', the spirometer can't see it: it misses the air that never leaves.

## Test yourself

Think of the answer, then tap the question to see it.

## Why can't spirometry measure functional residual capacity?

FRC contains the residual volume, which never leaves the lung; a spirometer only records air moving in and out of the mouth.

## If VC is 4.6 L and RV is 1.2 L, what is TLC?

TLC = VC + RV = 5.8 L.

## Why does FRC fall in an obese patient lying flat?

Abdominal contents push the diaphragm into the chest and shrink the resting volume, mostly at the expense of expiratory reserve volume.

## What happens to RV with air trapping in COPD, and why?

It rises: lost elastic recoil lets small airways close early in expiration, trapping air behind them.

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## Related topics

- [Obstructive and restrictive patterns, and flow–volume loops](https://durgical.com/en/physiology/obstructive-and-restrictive-patterns-and-flow-volume-loops/)
- [Mechanics of breathing: pressures and muscles](https://durgical.com/en/physiology/mechanics-of-breathing-pressures-and-muscles/)
- [Dead space and alveolar ventilation](https://durgical.com/en/physiology/dead-space-and-alveolar-ventilation/)
- [Lung and chest wall compliance](https://durgical.com/en/physiology/lung-and-chest-wall-compliance/)

## Related conditions

- [Asthma](https://durgical.com/en/conditions/asthma/)
- [Chronic obstructive pulmonary disease (COPD)](https://durgical.com/en/conditions/chronic-obstructive-pulmonary-disease/)
- [Interstitial lung disease](https://durgical.com/en/conditions/interstitial-lung-disease/)
- [Obesity](https://durgical.com/en/conditions/obesity/)

[← All of physiology](https://durgical.com/en/physiology/)

Educational physiology content; not a diagnostic or treatment guide and no substitute for a health professional's advice. Values are approximate references. No doses are given.

Sources: [22.3 The Process of Breathing](https://openstax.org/books/anatomy-and-physiology-2e/pages/22-3-the-process-of-breathing) (OpenStax (Rice University), 2022); [Chronic Obstructive Pulmonary Disease (COPD)](https://www.ncbi.nlm.nih.gov/books/NBK559281/) (StatPearls (NCBI Bookshelf), 2024); [Asthma](https://www.ncbi.nlm.nih.gov/books/NBK430901/) (StatPearls (NCBI Bookshelf), 2024); Hall JE. Guyton and Hall Textbook of Medical Physiology, 13th ed. (ch. 38). Reference book used to check facts; its text and figures are not reproduced (Elsevier, 2016).

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Representative model, for educational purposes only.

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License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/physiology/lung-volumes-capacities-and-spirometry/