Step by Step
TV
Tidal volume — normal breathing
About 500 mL — the amount of air moved in and out during one normal, relaxed breath.
IRV/ERV
Inspiratory and expiratory reserve volumes
Inspiratory reserve volume (IRV, ~3000 mL) is the extra air you can inhale beyond a normal tidal breath with maximum effort. Expiratory reserve volume (ERV, ~1200 mL) is the extra air you can exhale beyond a normal tidal breath with maximum effort.
RV
Residual volume — air that never leaves
About 1200 mL — the air remaining in the lungs after maximum exhalation, which can never be expelled. Because it can't be exhaled, residual volume cannot be measured by standard spirometry; it requires body plethysmography or helium dilution instead.
Cap
Capacities — combinations of volumes
Vital capacity (VC) = TV + IRV + ERV (~4700 mL) — the maximum usable range of breathing. Total lung capacity (TLC) = VC + RV (~5900 mL). Functional residual capacity (FRC) = ERV + RV. Inspiratory capacity = TV + IRV.
Applied Walkthrough
1
A patient breathes normally, moving about 500 mL of air per breath — this is their tidal volume.
2
Asked to inhale as deeply as possible after a normal breath, they pull in an additional ~3000 mL beyond their tidal volume — this extra amount is their inspiratory reserve volume.
3
Asked to then exhale as forcefully as possible, they can push out roughly 1200 mL beyond a normal tidal exhalation — their expiratory reserve volume. Even after this maximal exhalation, about 1200 mL of air still remains trapped in the lungs — the residual volume, which simply cannot be measured with a standard spirometer.
4
Adding tidal volume, inspiratory reserve, and expiratory reserve together gives this patient's vital capacity (~4700 mL) — their maximum usable breathing range; adding the immeasurable residual volume on top gives their total lung capacity (~5900 mL).
Exam Application
Exams test whether you can define each of the four lung volumes and correctly combine them into the four lung capacities, and specifically whether you know that residual volume (and thus TLC and FRC, which include it) cannot be measured by standard spirometry.
⚠ Common Trap
The most common trap is trying to calculate total lung capacity or functional residual capacity using only spirometry — since both include residual volume, which spirometry can't measure directly, these two capacities require alternative methods like body plethysmography or helium dilution.
✓ Quick Self-Check
1. What is tidal volume, and what is its approximate value?
The air moved during one normal breath, approximately 500 mL.
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2. What is residual volume, and why can't it be measured by spirometry?
The air remaining in the lungs after maximum exhalation (~1200 mL); it can't be measured by spirometry because it's never actually exhaled.
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3. What is the formula for vital capacity?
VC = TV + IRV + ERV.
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4. What is the formula for total lung capacity?
TLC = VC + RV.
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5. What two methods can measure residual volume, since spirometry cannot?
Body plethysmography or helium dilution.
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