Lung Compliance and Elastance: Reading Pressure–Volume Changes

Lung compliance asks how much lung volume changes for a given change in distending pressure. Elastance describes the reciprocal relationship: how much pressure change is needed per unit volume change. This guide follows the ventilation and dead-space guide by asking what mechanical properties influence expansion.

For Indian undergraduate medical students and postgraduate revision. Updated 12 November 2026.

Learning objectives

  • Define compliance and elastance with units.
  • Calculate transpulmonary pressure from alveolar and pleural pressures.
  • Interpret a pressure–volume slope over a stated range.
  • Distinguish lung compliance, respiratory-system compliance and airway resistance.

1. Choose the correct pressure difference

The lung is distended by transpulmonary pressure, PL = Palv − Ppl: alveolar pressure minus pleural pressure. If both are expressed relative to atmospheric pressure, alveolar pressure of 0 and pleural pressure of −5 cm H2O give PL = +5 cm H2O.

Do not confuse this with the pressure difference between the airway opening and alveoli that drives airflow. A lung can remain distended at a moment of zero airflow.

2. Compliance is a change divided by a change

CL = ΔV/ΔPL, commonly expressed in L/cm H2O or mL/cm H2O. Use the difference between two volumes and the difference between their corresponding transpulmonary pressures, rather than dividing a single absolute volume by a pressure.

On a graph with pressure on the horizontal axis and volume on the vertical axis, a steeper local slope means greater compliance. The curve is not a straight line across all volumes; a value calculated over one interval should not be assumed to describe every interval.

3. Worked example with units and assumptions

Consider two static, no-flow measurement points on the same inflation limb. Assume alveolar pressure is 0 cm H2O at each point, pleural pressure is represented adequately, and the stated volume measurements are comparable. These are invented teaching measurements, not reference values.

  • Point A: volume 2.60 L; pleural pressure −5 cm H2O. Transpulmonary pressure = 0 − (−5) = 5 cm H2O.
  • Point B: volume 3.05 L; pleural pressure −8 cm H2O. Transpulmonary pressure = 0 − (−8) = 8 cm H2O.

ΔV = 3.05 − 2.60 = 0.45 L. ΔPL = 8 − 5 = 3 cm H2O.

CL = 0.45/3 = 0.15 L/cm H2O = 150 mL/cm H2O.

If another lung needs a 6 cm H2O increase in transpulmonary pressure for the same 0.45 L expansion over a comparable interval, its compliance is 75 mL/cm H2O: half as large.

4. Elastance reverses the ratio

EL = ΔPL/ΔV = 1/CL for the same interval and measurement conditions. In the first example, elastance is 3/0.45 ≈ 6.67 cm H2O/L. In the second, it is 6/0.45 ≈ 13.3 cm H2O/L.

A lower compliance therefore corresponds to a higher elastance. Keep the volume units consistent before taking a reciprocal: 1/150 in mL-based units is not 6.67 in L-based units.

5. What determines the curve?

Lung tissue and the surface tension at the alveolar air–liquid interface contribute to recoil. Surfactant from type II alveolar cells reduces surface tension and helps the lung expand. Inflation and deflation can follow different pressure–volume paths, termed hysteresis; compare measurements on the same limb when interpreting a slope.

“More compliant” is not a synonym for “healthier.” A lung that expands readily may have reduced elastic recoil. Conversely, a stiff lung needs a larger pressure change for a comparable expansion. A compliance number alone does not identify a disease or describe gas exchange.

6. Separate three mechanical questions

  • Lung compliance: how lung volume responds to a change in pressure across the lung.
  • Respiratory-system compliance: how the combined lung and chest wall respond; it is not automatically the lung-only value.
  • Airway resistance: the pressure difference required for airflow through the airways, usually expressed as pressure divided by flow.

During airflow, measured airway pressure can contain a resistive component as well as elastic pressure. Static assessment removes the airflow-related pressure drop. Do not label every rise in airway pressure as a fall in lung compliance.

7. Misconceptions to avoid

  • “Compliance is V/P.” Use changes over a specified interval.
  • “A negative pleural pressure makes transpulmonary pressure negative.” Subtract the signed pleural value from alveolar pressure.
  • “Compliance and elastance rise together.” They are reciprocals under matched conditions.
  • “One slope describes the whole curve.” State the volume range and inflation or deflation limb.

8. Revision questions with answers

  1. A volume change of 300 mL follows a transpulmonary-pressure change of 4 cm H2O. What is compliance? 75 mL/cm H2O.
  2. If alveolar pressure is +2 and pleural pressure is −6 cm H2O, what is transpulmonary pressure? +8 cm H2O.
  3. At fixed compliance, doubling the pressure change over an approximately linear interval does what? It doubles the predicted volume change within that approximation.
  4. Why specify no-flow conditions? To avoid treating airflow-related resistive pressure as an elastic pressure change.
  5. Can a combined lung-and-chest-wall value be assumed to equal lung compliance? No; the measurement boundaries differ.

9. Choose your next study resource

For focused reading, inspect the Compliance of the Lung chapter. For broader system study, compare the contents of Respiratory Physiology: A Clinically Integrated Approach. These are separate paid digital resources; check each listing's scope and current checkout total.

The free Homeostasis sample introduces the teaching style on another foundational topic; it is not a lung-compliance sample. Browse other free guides in the physiology resource hub.

References and further reading

These original revision notes teach a simplified mechanical model. Follow course teaching for the depth and terminology required by your curriculum.