Lung and chest wall compliance !

Concept
Exam priority: medium

Compliance is how much the lung inflates for each unit of pressure. A stiff lung takes more work to fill.

Updated 2026-11-01

Also called: Compliance, Pulmonary compliance, Elastance

See it in 3D: Pleura · Right lower lobe

What it is

Compliance is the change in volume per change in transpulmonary pressure (ΔV/ΔP). It depends on elastic fibres and, above all, on alveolar surface tension, which surfactant lowers. The lung tends to collapse and the chest wall to spring out; at FRC the two forces balance.

How it works, step by step

On inflation, elastic fibres stretch and the alveolar liquid surface grows.
Surface tension tends to close alveoli; surfactant lowers it, the more so the smaller the alveolus.
In fibrosis, stiff collagen demands more pressure per millilitre: compliance falls.
In emphysema the lung inflates easily but loses recoil and cannot push the air out.

Lung volumes and capacities

TLC 3.6 LVC 2.8 LFRC 1.4 LRV 0.8 LFEV1/FVC 0.87

Restrictive (pulmonary fibrosis, chest wall or neuromuscular disease, obesity): all volumes ↓ (TLC < 80% predicted); FEV1 and FVC fall together: FEV1/FVC normal or ↑.

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

Stiff lung: every volume shrinks.

Equations

ComplianceC = ΔV / ΔP

ΔV: change in volume
ΔP: change in transpulmonary pressure

Elastance is its inverse (1/C): how much pressure each millilitre costs.

Approximate values (adult)

Parameter	Value	Note
Lung compliance	≈ 200 mL/cmH₂O
Respiratory system compliance (lung + chest wall)	≈ 110 mL/cmH₂O
Static compliance on a ventilator	≈ 50–100 mL/cmH₂O	lower supine and intubated

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

Changes in compliance

Condition	Compliance	Why
Interstitial fibrosis	Low	Stiff collagen in the interstitium
ARDS, pulmonary edema	Low	Flooded alveoli and less surfactant
Emphysema	High	Destroyed elastic fibres
Obesity, kyphoscoliosis	Low (chest wall)	Stiff or loaded chest wall

Clinical notes

ARDS: very low compliance calls for small tidal volumes so healthy alveoli are not overstretched.
Premature newborn: without surfactant, alveoli collapse and compliance is low (respiratory distress syndrome).

Test yourself

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

Why does an emphysematous lung inflate easily but empty poorly?

With elastic fibres lost, compliance rises, but so does the loss of the recoil that drives expiration and holds small airways open.

What contributes more to lung elastic recoil: elastic fibres or surface tension?

Surface tension; that is why a lung inflated with saline needs much less pressure than one inflated with air.

If giving 500 mL raises transpulmonary pressure by 5 cmH₂O, what is the compliance?

500/5 = 100 mL/cmH₂O.

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See it in 3D

The structures of Durgical's 3D anatomy where it happens.

← All of 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 (OpenStax (Rice University), 2022); Acute Respiratory Distress Syndrome (StatPearls (NCBI Bookshelf), 2024); Interstitial Lung Disease (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.