Lung Volumes & Capacities

A perennial NEET PG favourite that bridges pure physiology with bedside pulmonology. Master four primary, non-overlapping volumes and the four capacities built by adding them, then layer on how each shifts in obstructive versus restrictive disease and how they are measured. The numbers and the spirometry patterns are tested almost every year.

Definition & basic framework

A lung volume is one of four discrete, mutually exclusive subdivisions of the gas in the lungs. A lung capacity is the sum of two or more volumes. By convention all values are quoted for a healthy 70 kg adult male; female values run roughly 20–25% lower.

The four primary volumes (they do not overlap, so they can be added):

Volume	Definition	Approx. value
Tidal volume (TV / V_T)	Air moved in or out in one quiet (normal) breath	~500 mL
Inspiratory reserve volume (IRV)	Extra air that can be forcibly inhaled above a normal tidal inspiration	~3000 mL
Expiratory reserve volume (ERV)	Extra air that can be forcibly exhaled below the end of a normal tidal expiration	~1100 mL
Residual volume (RV)	Air remaining in the lungs after a maximal forced expiration	~1200 mL

The four capacities (each is a sum of volumes):

Capacity	Components	Approx. value	One-line meaning
Inspiratory capacity (IC)	TV + IRV	~3500 mL	Max air inhaled from resting end-expiratory level
Functional residual capacity (FRC)	ERV + RV	~2300 mL	Air left after a normal tidal expiration
Vital capacity (VC)	TV + IRV + ERV (= IC + ERV)	~4600 mL	Max air exhaled after max inhalation
Total lung capacity (TLC)	All four volumes (VC + RV)	~5800 mL	Total gas at end of maximal inspiration

High-yield: Any "capacity" that contains residual volume cannot be measured by simple spirometry. That means RV, FRC, and TLC require body plethysmography, helium dilution, or nitrogen washout. Spirometry measures TV, IRV, ERV, IC and VC only.

A clean way to anchor the relationships:

RV + ERV → FRC → (+ IC) → TLC, and separately VC = TLC − RV.

Key derived relationships (must memorise)

1. TLC = VC + RV
2. FRC = ERV + RV
3. VC = IRV + TV + ERV = IC + ERV
4. IC = TV + IRV
5. TLC = IC + FRC

High-yield: FRC is the resting/equilibrium volume of the lung — the point at which the inward elastic recoil of the lung exactly balances the outward recoil of the chest wall. At FRC the net (transpulmonary) pressure favouring further collapse is zero, so it is the most stable reference point in respiratory physiology.

Physiological significance of each value

Functional residual capacity (FRC) — the single most examined concept here.

• Acts as an oxygen reservoir / buffer between breaths, preventing wide swings in alveolar and arterial PO₂/PCO₂ during the respiratory cycle.
• Determined by the balance of elastic recoil forces (lung pulling in, chest wall springing out).
• FRC increases with: upright/erect posture, emphysema/COPD (loss of recoil → hyperinflation), asthma (air-trapping), increasing age, PEEP.
• FRC decreases with: supine posture, obesity, pregnancy, restrictive lung disease (fibrosis), general anaesthesia (~15–20% fall), ARDS, and abdominal compartment problems.

High-yield: During general anaesthesia, FRC falls (loss of diaphragmatic tone + supine position + cephalad shift of diaphragm). This promotes basal atelectasis and worsens V/Q matching — a classic clinical-physiology MCQ.

Residual volume (RV) — cannot be exhaled; prevents alveolar collapse and allows continuous gas exchange even between breaths. RV increases with age and in obstructive disease (air-trapping); a rising RV/TLC ratio is a marker of hyperinflation.

Vital capacity (VC) — overall index of ventilatory capacity. Reduced in both restrictive disease (small lungs) and severe obstructive disease (air-trapping reduces the volume that can be exhaled). Falls with respiratory muscle weakness (e.g., Guillain–Barré, myasthenia) — serial VC is the bedside parameter used to decide intubation in neuromuscular respiratory failure (threshold roughly <15 mL/kg or <1 L).

Closing capacity (CC) — the lung volume at which dependent small airways begin to close (= closing volume + RV). Normally CC < FRC, so airways stay open during tidal breathing. CC rises with age and in smokers; when CC exceeds FRC, airways close during tidal breathing → hypoxaemia. This is why the elderly and supine patients desaturate more readily.

Measurement: how each value is obtained

Spirometry directly records volume changes during breathing and gives TV, IRV, ERV, IC and VC. It cannot measure RV (the patient cannot exhale it), and therefore cannot give FRC or TLC.

To capture the residual-volume-containing capacities, use one of three techniques:

Method	Principle	Notes
Helium dilution	Closed circuit; inert helium equilibrates only in communicating (ventilated) airspaces; FRC calculated from dilution: C₁V₁ = C₂(V₁+V₂)	Measures only ventilated gas
Nitrogen washout	Patient breathes 100% O₂; exhaled N₂ collected; measures communicating gas	Also measures only ventilated gas
Body plethysmography	Boyle's law (P₁V₁ = P₂V₂) in a sealed box; measures all intrathoracic gas	Gold standard; includes non-communicating gas

High-yield: In conditions with air-trapping or non-communicating gas (bullae, severe emphysema, pneumothorax), helium dilution and nitrogen washout UNDERESTIMATE FRC/TLC because trapped gas never equilibrates with helium. Body plethysmography measures all intrathoracic gas and gives the true (higher) value. A plethysmography–dilution discrepancy quantifies the trapped gas.

Helium dilution flow (stepwise):

1. Spirometer is filled with a known volume and known helium concentration (C₁, V₁).
2. Patient connected at end of normal tidal expiration (so the measured starting lung volume is FRC).
3. Patient rebreathes until helium equilibrates (concentration falls to C₂).
4. Lung volume V₂ (= FRC) solved from C₁V₁ = C₂(V₁ + V₂).
5. RV = FRC − ERV; TLC = FRC + IC.

Obstructive vs restrictive patterns — the clinical core

This single comparison generates the largest share of NEET PG questions on the topic.

Parameter	Obstructive (COPD, asthma)	Restrictive (fibrosis, kyphoscoliosis, neuromuscular)
FEV₁	↓↓ (markedly)	↓ (proportionate)
FVC	Normal or ↓	↓↓
FEV₁/FVC ratio	↓ (<0.7)	Normal or ↑ (≥0.7, often >0.8)
TLC	Normal or ↑ (hyperinflation)	↓
RV	↑↑ (air-trapping)	↓ (or normal)
RV/TLC	↑	Normal or ↑
FRC	↑	↓
DLCO	↓ (emphysema), normal (asthma/chronic bronchitis)	↓ (parenchymal e.g. fibrosis), normal (chest-wall/neuromuscular)

High-yield: The FEV₁/FVC ratio is the gatekeeper. Low ratio (<0.7) = obstructive; normal or high ratio with low FVC and low TLC = restrictive. TLC is what confirms restriction (FVC alone can fall in obstruction too).

Diagnostic flow for a PFT question:

Step 1 → Look at FEV₁/FVC. If <0.7 → obstructive, stop and grade severity by FEV₁. Step 2 → If ratio normal/high → check TLC. Low TLC → restrictive. Step 3 → In obstruction, give a bronchodilator: ≥12% AND ≥200 mL rise in FEV₁ → reversible (asthma); minimal change → fixed (COPD). Step 4 → Use DLCO to subdivide: low in emphysema and fibrosis; normal in asthma, chronic bronchitis, and chest-wall/neuromuscular restriction.

High-yield: A scooped/concave expiratory limb of the flow-volume loop = obstruction. A tall, narrow loop shifted toward TLC with preserved shape = restriction. Variable extrathoracic obstruction flattens the inspiratory limb (e.g., vocal cord dysfunction); variable intrathoracic obstruction flattens the expiratory limb; fixed obstruction (e.g., tracheal stenosis) flattens both (box-shaped loop).

Spirometry-derived dynamic indices

• FVC — forced vital capacity: max air forcibly exhaled after max inspiration.
• FEV₁ — volume exhaled in the first second of FVC manoeuvre; the workhorse for grading obstruction.
• FEF 25–75% (MMEF) — mid-expiratory flow; reflects small-airway function and is an early, sensitive marker of small-airway obstruction (falls before FEV₁/FVC in early COPD/asthma).
• PEFR — peak expiratory flow rate; effort-dependent; used for asthma home monitoring.
• MVV — maximal voluntary ventilation.

High-yield: FEF 25–75% is the earliest spirometric abnormality in small-airway disease and is effort-independent (unlike PEFR which is effort-dependent).

Factors that change lung volumes — quick recall

• Posture: Supine → FRC and ERV fall (abdominal contents push diaphragm cephalad); TV unchanged.
• Body size/height: All volumes scale with height; tall > short.
• Sex: Female volumes ~20–25% lower than male.
• Age: RV and FRC ↑, VC ↓, TLC roughly unchanged (loss of recoil + stiffer chest wall); closing capacity ↑.
• Pregnancy & obesity: FRC and ERV ↓ (diaphragm pushed up); TV ↑ in pregnancy (progesterone-driven hyperventilation).
• Exercise: TV ↑ encroaching on both IRV and ERV; respiratory rate ↑.
• Athletic training/altitude acclimatisation: VC tends to be higher.

Dead space (commonly bundled with this topic)

• Anatomical dead space ≈ 150 mL — conducting airways (nose to terminal bronchioles) where no gas exchange occurs. Roughly 2 mL/kg (≈ 1 mL per pound).
• Alveolar dead space — ventilated alveoli that are not perfused (e.g., pulmonary embolism); ~0 in healthy lungs.
• Physiological dead space = anatomical + alveolar; markedly increased in PE.
• Bohr equation: V_D/V_T = (PaCO₂ − P_ECO₂) / PaCO₂.
• Alveolar ventilation (V_A) = (TV − dead space) × respiratory rate. Increasing tidal volume is more efficient for alveolar ventilation than increasing rate, because dead space is "wasted" once per breath.

High-yield: Of TV (~500 mL), only ~350 mL reaches alveoli; ~150 mL fills dead space. Rapid shallow breathing wastes a larger fraction in dead space and lowers effective alveolar ventilation — relevant to ventilator strategy and panic/tachypnoea questions.

Complications / clinical correlates

• Hyperinflation (↑RV, ↑FRC, ↑TLC) in COPD flattens the diaphragm, shortens its fibres, and impairs its mechanical efficiency → dyspnoea and the Hoover's sign (paradoxical inward movement of lower ribs).
• Dynamic hyperinflation & auto-PEEP during exacerbations or exercise raise FRC further and increase the work of breathing.
• Falling VC in neuromuscular disease (GBS, myasthenic crisis) heralds respiratory failure before hypoxia appears — monitor serial VC.
• Reduced FRC under anaesthesia/obesity/ARDS → atelectasis, shunt, hypoxaemia; managed with PEEP and recruitment.

Key differentials / look-alike concepts

• Obstructive vs restrictive — resolved by FEV₁/FVC ratio + TLC (see table).
• Helium dilution vs plethysmography discrepancy — points to trapped/non-communicating gas (bullous emphysema).
• VC vs FVC — VC (slow, relaxed) may exceed FVC in obstruction because forced effort causes premature airway collapse and air-trapping; this gap itself suggests obstruction.
• FRC vs closing capacity — when CC > FRC, tidal airway closure and desaturation occur.

Recently asked / exam angle

• "Which lung capacity cannot be measured by spirometry?" → FRC (and RV, TLC). Classic single-best-answer.
• "Method that measures both communicating and non-communicating gas / true TLC?" → Body plethysmography.
• "In emphysema, helium dilution gives a TLC lower than plethysmography because…" → trapped gas does not equilibrate.
• "FRC = ?" → ERV + RV; and "the volume at end of normal expiration where recoil forces balance."
• Flow-volume loop image: identify fixed upper-airway obstruction (both limbs flattened) vs variable extrathoracic (inspiratory limb flattened).
• "Earliest spirometric change in small-airway disease" → FEV₁/FVC drop in ratio is classic, but FEF 25–75% is the earliest/most sensitive.
• "FRC change in supine position / anaesthesia / pregnancy" → decreases.
• Calculation MCQ using C₁V₁ = C₂(V₁+V₂) for helium dilution FRC.
• "Anatomical dead space value" → ~150 mL (2 mL/kg).

Mnemonics:

• "FRC = Empty Room" → ERV + RV (R sounds like Room).
• "VC has everything but the Residue" → VC = TLC − RV.
• Capacities are combinations: any word ending in "capacity" = ≥2 volumes added; the standalone "volumes" (TV, IRV, ERV, RV) are the building blocks.

Rapid revision

1. Four volumes: TV (500), IRV (3000), ERV (1100), RV (1200); they never overlap, so they add up.
2. FRC = ERV + RV (~2300 mL) = resting lung volume where lung and chest-wall recoil balance.
3. TLC = VC + RV (~5800 mL); VC = IC + ERV (~4600 mL); IC = TV + IRV.
4. Spirometry cannot measure RV, FRC, or TLC — anything containing RV needs dilution or plethysmography.
5. Body plethysmography = gold standard; measures all intrathoracic gas including trapped/non-communicating gas.
6. Helium dilution & N₂ washout underestimate lung volumes when air-trapping is present.
7. Obstructive: FEV₁/FVC ↓ (<0.7), RV ↑, FRC ↑, TLC normal/↑. Restrictive: ratio normal/↑, TLC ↓, FVC ↓.
8. FEV₁/FVC ratio is the gatekeeper; TLC confirms restriction.
9. FRC falls supine, in obesity, pregnancy, anaesthesia, ARDS; rises in COPD/emphysema, age, PEEP.
10. FEF 25–75% = earliest, effort-independent marker of small-airway disease.
11. Anatomical dead space ≈ 150 mL (2 mL/kg); alveolar ventilation = (TV − dead space) × RR.
12. Serial VC guides intubation in neuromuscular respiratory failure (~<15 mL/kg or <1 L).