Lung volumes and capacities
Also known as: spirometry, tidal volume, vital capacity, functional residual capacity, FRC, residual volume, total lung capacity, FEV1, FVC
The four standard lung volumes (TV, IRV, ERV, RV) and the capacities formed by adding them (IC, FRC, VC, TLC), measured by spirometry and gas-dilution or plethysmographic methods.
Overview
Volumes (young adult male, approximate): tidal volume (TV) ≈500 mL — air moved in a quiet breath; inspiratory reserve volume (IRV) ≈3000 mL — extra air that can be inhaled after a normal inspiration; expiratory reserve volume (ERV) ≈1100–1200 mL — extra air that can be exhaled after a normal expiration; residual volume (RV) ≈1200 mL — air that remains after maximal expiration (it keeps alveoli open and buffers alveolar gas between breaths).
Capacities: inspiratory capacity IC = TV + IRV ≈3500 mL; functional residual capacity FRC = ERV + RV ≈2300–2400 mL — the resting end-expiratory volume, where inward lung recoil equals outward chest-wall recoil; vital capacity VC = IRV + TV + ERV ≈4600–4800 mL; total lung capacity TLC = VC + RV ≈5800–6000 mL. Values are ≈20–25% smaller in women and depend on height, age, sex and ethnicity, so results are expressed as % predicted or z-scores.
Dynamic spirometry: forced vital capacity (FVC) and the volume exhaled in the first second (FEV1); normal FEV1/FVC ≈0.75–0.8. RV, FRC and TLC cannot be measured by spirometry and require helium dilution, nitrogen washout or body plethysmography (which also measures trapped gas).
- Location
Whole-lung concept.
Parts & subdivisions
- Tidal volume (TV)≈500 mL (≈6–8 mL/kg).
- Inspiratory reserve volume (IRV)≈3000 mL.
- Expiratory reserve volume (ERV)≈1100–1200 mL; falls when supine and in obesity and pregnancy.
- Residual volume (RV)≈1200 mL; rises with age and in obstruction.
- Functional residual capacity (FRC)ERV + RV ≈2300–2400 mL; the body's O2 reservoir during apnoea (basis of pre-oxygenation).
- Vital capacity (VC) and total lung capacity (TLC)≈4600–4800 mL and ≈5800–6000 mL.
Function
- Quantify ventilatory capacity and classify lung disease.
- FRC provides an O2 store and keeps alveolar gas stable between breaths.
Clinical significance
Obstructive vs restrictive patterns
Obstructive (asthma, COPD, bronchiectasis, CF): FEV1/FVC < 0.7, ↓FEV1, normal or ↓FVC, ↑RV/FRC/TLC (air trapping, hyperinflation); a scooped-out expiratory flow–volume loop. Restrictive (interstitial lung disease, chest-wall deformity, neuromuscular weakness, obesity): ↓TLC (< 80% predicted), ↓FVC, normal or ↑FEV1/FVC. DLCO distinguishes parenchymal (↓) from extraparenchymal (normal) restriction.
Pre-oxygenation
Breathing 100% O2 before anaesthetic induction replaces the nitrogen in the FRC with oxygen, extending safe apnoea time from ≈1 to ≈8 minutes in healthy adults — less in obesity, pregnancy and children, who have a small FRC relative to O2 consumption.
Mnemonics
Capacities
A capacity is always the sum of two or more volumes. "FRC = ERV + RV — FRC is what's left in the End Recoil position."
Test yourself
Functional residual capacity is the sum of which two volumes?
- Tidal volume + inspiratory reserve volume
- Expiratory reserve volume + tidal volume
- Expiratory reserve volume + residual volume
- Inspiratory reserve volume + residual volume
Answer: Expiratory reserve volume + residual volume
FRC = ERV + RV (≈1100–1200 + 1200 ≈ 2300–2400 mL) — the volume remaining after a normal passive expiration, when inward lung recoil balances outward chest-wall recoil. Because it contains RV, it cannot be measured by simple spirometry.
Which of the following CANNOT be measured by simple spirometry?
- Vital capacity
- Residual volume
- FEV1
- Inspiratory capacity
Answer: Residual volume
Residual volume is the air that remains after a maximal expiration and never leaves the lungs, so a spirometer cannot record it. RV — and therefore FRC and TLC — is measured by helium dilution, nitrogen washout or body plethysmography.
Tidal volume is 500 mL, anatomical dead space 150 mL and respiratory rate 12/min. What is the alveolar ventilation?
- 6.0 L/min
- 1.8 L/min
- 7.8 L/min
- 4.2 L/min
Answer: 4.2 L/min
Alveolar ventilation = (VT − VD) × f = (500 − 150) × 12 = 4200 mL/min. Minute ventilation is 6.0 L/min. Rapid shallow breathing wastes a larger fraction of each breath on dead space.
Compiled from standard anatomy and physiology references — see the Respiratory System sources. For education only; not medical advice.