Lung compliance and elastic recoil

Also known as: compliance, elastance, law of Laplace, surface tension, airway resistance, work of breathing, hysteresis

Compliance (ΔV/ΔP) describes how easily the lungs and chest wall stretch; it is set by elastic fibres and, above all, alveolar surface tension, which surfactant reduces.

Overview

Lung compliance≈200 mL/cmH2O

Total respiratory system≈100 mL/cmH2O

Recoil due to surface tension≈⅔

Compliance C = ΔV/ΔP: lungs ≈200 mL/cmH2O, chest wall ≈200 mL/cmH2O, total respiratory system ≈100 mL/cmH2O (the two act in series, so their elastances add). Its reciprocal, elastance, is the tendency to recoil. Lung recoil comes from elastic and collagen fibres (≈⅓) and surface tension at the alveolar air–liquid interface (≈⅔) — shown by the fact that a saline-filled lung inflates far more easily than an air-filled one. The pressure–volume curve shows hysteresis: volume at a given pressure is greater during deflation than inflation, largely because of surfactant behaviour.

Law of Laplace: P = 2T/r. Without surfactant, small alveoli would generate higher collapsing pressures and empty into large ones; surfactant lowers T in proportion to its surface concentration, which rises as an alveolus shrinks, so alveoli of different sizes coexist stably. Compliance is lowest at very low and very high volumes, so tidal breathing normally takes place on the steep middle part of the curve around FRC.

Airway resistance (R ∝ 1/r⁴, Poiseuille) is the other major load: most resides in the medium-sized bronchi; the small airways contribute little because of their enormous combined cross-section. Resistance falls as lung volume increases (radial traction), and is raised by bronchoconstriction, mucus, oedema and airway collapse. Work of breathing reflects both elastic and resistive loads.

Location

Lung parenchyma, alveolar surface and chest wall.

Function

Determines the pressure (and muscular work) needed for each breath.

Elastic recoil drives passive expiration and sets FRC.

Physiology

Static compliance is measured during breath-holds at several volumes (oesophageal pressure approximates intrapleural pressure); dynamic compliance during breathing additionally reflects airway resistance and falls with rising respiratory rate when small airways are diseased.

Clinical significance

Increased compliance — emphysema and ageing

Destruction of elastic tissue makes the lung easy to inflate but poor at recoiling: low expiratory driving pressure, airway collapse, air trapping and hyperinflation (barrel chest, flattened diaphragms).

Decreased compliance — fibrosis, oedema, ARDS, IRDS

Stiff lungs need large pressure swings: small tidal volumes, rapid shallow breathing, high work of breathing and reduced lung volumes (restrictive pattern). Obesity, kyphoscoliosis and circumferential burns reduce chest-wall compliance.

Test yourself

According to the law of Laplace, why would small alveoli collapse into larger ones in the absence of surfactant?

Small alveoli have a lower surface tension
The collapsing pressure is inversely proportional to alveolar radius
Large alveoli have a higher collapsing pressure
Surface tension is proportional to alveolar volume

Answer: The collapsing pressure is inversely proportional to alveolar radius

P = 2T/r. At equal surface tension, a smaller radius generates a greater collapsing pressure, so a small alveolus would empty into a larger one. Surfactant lowers T more in small alveoli (where its molecules are compressed together), equalising pressures and stabilising alveoli.