Lung Volumes, Capacities & Measurement

The spirogram

A volume cannot be subdivided. A capacity is two or more volumes added together. Everything below follows from that.

Volume	Definition	Normal adult
Tidal volume (V_T)	Air moved in or out during quiet breathing	500 mL (≈7 mL/kg)
Inspiratory reserve (IRV)	Maximum extra volume inspired after a normal inspiration	3000 mL
Expiratory reserve (ERV)	Maximum extra volume expired after a normal expiration	1100 mL
Residual volume (RV)	Air remaining after maximal expiration	1200 mL

Capacity	Formula	Normal adult
Inspiratory capacity (IC)	V_T + IRV	3500 mL
Functional residual capacity (FRC)	ERV + RV	2300 mL
Vital capacity (VC)	IRV + V_T + ERV	4600 mL
Total lung capacity (TLC)	VC + RV = FRC + IC	5800 mL

500

Vₜ (mL)

3000

IRV (mL)

1100

ERV (mL)

1200

RV (mL)

If it contains residual volume, simple spirometry cannot measure it.
Can measure: V_T, IRV, ERV, IC and VC.
Cannot measure: RV, FRC and TLC — because RV is, by definition, the gas you cannot breathe out and therefore cannot be captured in a spirometer.

Method	Principle	Limitation
Helium dilution	Closed circuit; an inert, insoluble tracer equilibrates through the communicating lung. Helium is detected by thermal conductivity	Underestimates when gas is trapped or poorly ventilated — the tracer never reaches it
Nitrogen washout	The patient breathes 100% oxygen; all resident nitrogen is washed out and its total volume measured	Also underestimates with trapped or poorly communicating gas
Body plethysmography	Uses Boyle's law (P₁V₁ = P₂V₂) in a sealed box to measure total thoracic gas volume	Includes trapped gas — so it is the method of choice in severe obstruction, and reads higher than the dilution methods

FRC = (C₁ × V₁) / C₂ − V₁
C₁ = initial helium concentration · V₁ = initial spirometer volume · C₂ = final (equilibrated) helium concentration.
Then RV = FRC − ERV and TLC = FRC + IC.

In a patient with emphysema and large bullae, plethysmography gives a much larger FRC than helium dilution. The difference between the two is the volume of trapped gas — a neat way of quantifying it, and a favourite question.

Gas	Measured by	Used in
Helium	Thermal conductivity; mass spectrometry	Helium dilution
Nitrogen	Mass spectrometry / nitrogen analyser	Nitrogen washout
Oxygen	Paramagnetic analyser; fuel cell; polarographic (Clark) electrode	Routine gas monitoring
Argon, xenon	Mass spectrometry	Tracer studies
CO₂ and volatile agents	Infrared absorption	Capnography, agent monitoring

Infrared absorption needs a molecule made of two DIFFERENT atoms. So it works for CO₂, N₂O and volatiles — but never for oxygen, nitrogen, helium or argon.
Oxygen is measured paramagnetically because it has two unpaired electrons and is attracted into a magnetic field.

Closing capacity = residual volume + closing volume.
Closing volume is the lung volume at which small dependent airways begin to close during expiration, because they lack cartilage and depend on radial traction to stay open.
Closing capacity rises with age. It exceeds FRC at about age 44 supine and about age 66 erect — from that point onwards some airways close during normal tidal breathing, causing shunt and explaining the age-related fall in PaO₂.
FRC falls with supine position, anaesthesia (by about 20%), obesity and pregnancy — all of which push FRC below closing capacity and worsen oxygenation. Closing capacity itself is unchanged by position.
Measured by the single-breath nitrogen washout (Fowler's method) or a tracer bolus technique.