Lung Compliance and Elasticity

Lung compliance is the change in lung volume produced by a unit change in distending (transpulmonary) pressure, and it expresses the elastic ease with which the lung can be inflated. Elasticity is the complementary property — the tendency of the stretched lung to recoil back toward a smaller volume — and together they set the relationship between the pressure applied to the lung and the volume it holds.

Definition

Lung compliance is the ratio of a change in lung volume to the change in transpulmonary pressure that produces it (the slope of the pressure-volume curve); lung elasticity, or elastic recoil, is the inward pressure the distended lung generates as it tends to return to a lower volume.

Scope

This topic covers the elastic pressure-volume behaviour of the lung and chest wall, the determinants of that elasticity (tissue elastic fibres and alveolar surface tension), the role of surfactant in lowering surface tension, and the way compliance is read from the slope of the pressure-volume curve. It is a reference account of mechanical properties, not clinical guidance.

Core questions

• How is compliance defined as the slope of the pressure-volume curve, and how does it differ from elastance?
• What contributes lung elastic recoil — tissue elastic fibres versus alveolar surface tension?
• How does surfactant alter surface tension and thereby compliance and alveolar stability?
• How do the elastic properties of the lung and chest wall combine to set resting lung volume?

Key concepts

• Compliance and elastance
• Pressure-volume curve
• Elastic recoil
• Hysteresis
• Surface tension
• Pulmonary surfactant
• Specific compliance

Key theories

Elastic continuum model of the lung

Mead, Takishima and Leith treated the lung as an interdependent elastic structure in which the recoil pressure at each region depends on the local volume to which it is stretched, accounting for how stresses and expansion distribute across the parenchyma.

Surface tension and surfactant contribution to recoil

A substantial part of lung elastic recoil arises from surface tension at the alveolar air-liquid interface; pulmonary surfactant lowers and stabilizes this tension, increasing compliance and preventing small alveoli from collapsing.

Mechanisms

The relaxed lung's volume is determined by the balance between the distending transpulmonary pressure and its elastic recoil. Plotting volume against transpulmonary pressure yields a sigmoid pressure-volume curve whose slope is the compliance; the curve differs on inflation and deflation (hysteresis), largely because of surface-tension behaviour at the alveolar interface. Two factors generate recoil: the elastic fibres of lung tissue and the surface tension of the thin liquid film lining the alveoli. Pulmonary surfactant reduces surface tension, which raises compliance and, because the effect is greater at low volumes, stabilizes alveoli of different sizes. Because the lung recoils inward and the chest wall tends to recoil outward, their opposing elastic forces meet at the resting lung volume.

Clinical relevance

Compliance and recoil define how readily the lung inflates, and altered elasticity underlies broad mechanical categories of lung disease — for example loss of recoil increases compliance, while fibrosis or surfactant deficiency reduces it. These concepts inform the interpretation of pressure-volume measurements and the rationale behind lung-protective ventilation. This entry describes physiological properties and is not a basis for individual diagnosis or treatment.

Evidence & guidelines

The quantitative description of lung elasticity rests on classic respiratory-mechanics studies and is consolidated in physiology handbooks and texts; clinical measurement of compliance is performed within standardized pulmonary-function and respiratory-mechanics frameworks.

History

Systematic measurement of lung elasticity developed alongside techniques for estimating pleural pressure in the mid-twentieth century, allowing the pressure-volume curve to be defined and compliance to be quantified. Mead and colleagues' modelling of stress distribution and the recognition of surface tension's role refined the understanding of what makes the lung elastic, work synthesized in the Handbook of Physiology volume on breathing mechanics.

Key figures

• Jere Mead
• Peter Macklem
• John B. West

Seminal works

• mead-1970
• mead-1967

Frequently asked questions

What is the difference between compliance and elastance?

Compliance is the volume gained per unit increase in distending pressure, while elastance is its reciprocal — the pressure needed per unit volume change. A highly compliant lung is easy to inflate; a stiff, high-elastance lung resists inflation.

Why is pulmonary surfactant important for lung mechanics?

Surfactant lowers the surface tension of the alveolar liquid film, which increases lung compliance and, because its effect is greater at low lung volumes, helps keep small alveoli from collapsing and stabilizes alveoli of different sizes.