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Lung Compliance Explained: Definition, Formula, Static vs Dynamic

November 3, 2026

Quick answer: Lung compliance is how easily the lungs stretch: the change in lung volume for a given change in transpulmonary pressure, written C = ΔV / ΔP. High compliance means the lungs inflate easily (as in emphysema); low compliance means they are stiff (as in pulmonary fibrosis, pulmonary edema or ARDS). StatPearls gives the normal total compliance of both lungs in an adult as approximately 200 mL/cm H₂O. On a ventilator, static compliance = tidal volume / (plateau pressure − PEEP) and dynamic compliance = tidal volume / (peak pressure − PEEP).

In Turkish lectures you will hear the same idea as kompliyans, and it shows up in physiology, pulmonology, anesthesia and intensive care. This article walks through the definition, the formulas, a worked example and the clinical patterns you are expected to recognize.

The definition, step by step

Compliance is a general property of elastic structures: how much volume changes when pressure changes. For the lungs, the pressure that matters is the transpulmonary pressure, the difference between alveolar pressure and pleural pressure:

Transpulmonary pressure = P(alveolar) − P(pleural)

So:

Lung compliance (C) = ΔV / ΔP(transpulmonary)

Units are volume per pressure, usually mL/cm H₂O.

Two related ideas help it stick:

  • Elastance is the inverse of compliance (E = 1/C). A stiff lung has high elastance and low compliance. Elastance describes the tendency to recoil back to resting volume.
  • Compliance is the slope of the pressure–volume curve. Plot lung volume against pressure: a steep slope means high compliance; a flat slope means low compliance.

Lung, chest wall and total respiratory system

The lungs sit inside the chest wall, and both must stretch when you inspire. The lungs and chest wall act in series, so their elastances add: 1/C(total) = 1/C(lung) + 1/C(chest wall). In practice this means a problem in either one lowers the compliance you measure at the airway. Conditions that stiffen the chest wall rather than the lung (for example, severe kyphoscoliosis) still reduce total respiratory system compliance.

Static vs dynamic compliance

This distinction matters most in mechanically ventilated patients.

Static compliance Dynamic compliance
When it's measured With no airflow (e.g. an inspiratory pause) During breathing, while air is flowing
What it reflects Elastic properties of lung and chest wall only Elastic properties plus airway resistance
Ventilator formula Vt / (Pplat − PEEP) Vt / (PIP − PEEP)

Vt = tidal volume; Pplat = plateau pressure; PIP = peak inspiratory pressure; PEEP = positive end-expiratory pressure.

Because peak pressure includes the pressure needed to push gas through the airways, PIP is higher than Pplat, so dynamic compliance is normally lower than static compliance.

Worked example

A ventilated patient receives a tidal volume of 500 mL with PEEP 5 cm H₂O. The peak inspiratory pressure is 30 cm H₂O and the plateau pressure is 25 cm H₂O.

  • Static compliance = 500 / (25 − 5) = 500 / 20 = 25 mL/cm H₂O
  • Dynamic compliance = 500 / (30 − 5) = 500 / 25 = 20 mL/cm H₂O

The numbers here only illustrate the arithmetic; interpret real values against the patient's own trend and your local protocols.

Reading the pattern

Comparing peak and plateau pressures is a classic exam question:

  • Peak pressure rises, plateau pressure stays the same: dynamic compliance falls, static doesn't. The problem is airway resistance, such as bronchospasm, secretions or a kinked or obstructed tube.
  • Peak and plateau pressures both rise: static compliance falls. The problem is reduced respiratory system compliance, for example pulmonary edema, ARDS, atelectasis or a pneumothorax.

What lowers compliance (stiff lungs)

  • Pulmonary fibrosis: elastic lung tissue is replaced with stiffer fibrous tissue.
  • Pulmonary edema and ARDS: fluid-filled and inflamed alveoli resist expansion.
  • Atelectasis: collapsed alveoli don't contribute to volume change.
  • Surfactant deficiency (for example, neonatal respiratory distress syndrome): higher surface tension makes alveoli harder to open.
  • Chest wall restriction (such as kyphoscoliosis) reduces total respiratory system compliance even if the lung itself is normal.

Low compliance means the patient (or ventilator) needs more pressure to move the same volume, so the work of breathing increases. Patients often breathe with smaller, faster breaths.

What raises compliance (floppy lungs)

  • Emphysema: destruction of alveolar walls and loss of elastic recoil make the lung easier to inflate but harder to empty, which contributes to air trapping.
  • Aging: structural changes in lung elastic tissue increase compliance.

High compliance is not "better" compliance. A lung that inflates easily but doesn't recoil empties poorly.

Why surfactant matters

Type II alveolar cells produce pulmonary surfactant, which lowers surface tension at the air–liquid interface in the alveoli. Lower surface tension makes alveoli easier to inflate, so surfactant increases compliance and helps keep small alveoli from collapsing. The Turkish glossary entry on surfactans pulmonalis covers this in more detail.

Hysteresis: why inflation and deflation differ

If you plot the pressure–volume curve during inflation and then deflation, the two lines don't overlap. At the same pressure, lung volume is higher during deflation than during inflation. This gap is called hysteresis, and surface forces in the alveoli are a major contributor. Exam questions often show this loop and ask you to identify the inflation and deflation limbs.

How to study this for exams

  1. Memorize one formula and derive the rest. If you know C = ΔV/ΔP, the ventilator formulas just plug in which pressures you use.
  2. Draw the pressure–volume curve for normal, fibrotic and emphysematous lungs side by side. The slopes tell the whole story.
  3. Practice the peak vs plateau pattern with a few made-up ventilator readings until you can name the problem in five seconds.
  4. Review with spaced flashcards. Put the formulas, the "lowers vs raises" lists and the peak/plateau rule on separate cards, and revisit them over several weeks.

Related reading: pulmonary ventilation, vital capacity, and the Turkish entry on kompliyans.

Turn your respiratory physiology lecture into flashcards

Finito Medicine turns your own lecture slides and PDFs into SM-2 spaced-repetition flashcards and quizzes, and the AI chat can re-explain a concept like compliance until it clicks, in English or Turkish. Start free in your browser or download the app.

FAQ

What is lung compliance in simple terms?

How easily the lungs stretch. It is the change in lung volume produced by a given change in pressure.

What is the formula for lung compliance?

C = ΔV / ΔP, where ΔP is the change in transpulmonary pressure (alveolar minus pleural pressure).

What is the difference between static and dynamic compliance?

Static compliance is measured with no airflow and reflects only elastic properties. Dynamic compliance is measured during airflow and also reflects airway resistance, so it is normally lower.

Is compliance increased or decreased in emphysema?

Increased, because loss of elastic recoil makes the lungs easier to inflate.

What does kompliyans mean in Turkish medicine?

It's the Turkish term for compliance. In respiratory physiology it means lung (or respiratory system) compliance as described above.

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