Lung Compliance and Elastic Recoil

Core Idea

Lung compliance—the change in lung volume per unit change in pressure—depends on elastic recoil properties conferred by collagen and elastin, and surface tension at the air-liquid interface reduced by pulmonary surfactant. Reduced compliance increases work of breathing and contributes to restrictive lung diseases.

How It's Best Learned

Compare pressure-volume curves for normal lungs vs. lungs with reduced compliance. Discuss why surfactant reduces surface tension and why its absence (in respiratory distress syndrome) causes compliance to fall.

Explainer

From your respiratory system overview, you know that breathing requires the respiratory muscles to generate pressure changes that move air in and out of the lungs. But how much pressure is needed to inflate the lungs by a given volume? That question is answered by lung compliance — defined as the change in lung volume per unit change in transmural pressure (ΔV/ΔP). High compliance means the lung inflates easily with little pressure; low compliance means the lung is stiff and resists expansion.

Two forces determine compliance. The first is the elastic tissue of the lung — networks of collagen and elastin fibers woven through the alveolar walls and around airways. Elastin stretches easily and snaps back, like a rubber band; collagen is stiffer and limits overexpansion, like a safety strap. Together they create elastic recoil, the tendency of the lung to collapse inward after being stretched. This is why the lungs don't simply stay inflated when you stop breathing in — the elastic fibers pull them back toward their resting volume. In diseases like emphysema, destruction of elastic fibers reduces recoil, making the lung very compliant (easy to inflate) but unable to deflate efficiently, trapping air inside.

The second — and often more important — force is surface tension at the air-liquid interface lining each alveolus. Every alveolus is coated with a thin film of water, and the cohesive forces between water molecules at this film's surface create an inward-directed tension that tends to collapse the alveolus, much like a soap bubble trying to shrink. According to the Law of Laplace, the collapsing pressure generated by surface tension is higher in smaller alveoli (P = 2T/r). Without compensation, small alveoli would collapse into larger ones, and the enormous total surface tension across 300 million alveoli would make the lungs extremely stiff — requiring dangerously high pressures to inflate.

The solution is pulmonary surfactant, a mixture of phospholipids (mainly dipalmitoylphosphatidylcholine) and proteins secreted by type II alveolar cells. Surfactant molecules sit at the air-liquid interface with their hydrophobic tails pointing toward the air and their hydrophilic heads in the water, disrupting the cohesive forces between water molecules and dramatically reducing surface tension. Crucially, surfactant's effect is concentration-dependent: as an alveolus shrinks during expiration, surfactant molecules are compressed together, reducing surface tension more — which prevents collapse. As it expands during inspiration, surfactant molecules spread apart, allowing surface tension to rise slightly — which prevents overexpansion. This dynamic behavior stabilizes alveoli of different sizes and reduces the overall work of breathing by roughly two-thirds. Premature infants who lack sufficient surfactant develop neonatal respiratory distress syndrome: their lungs are so non-compliant that each breath requires enormous effort, and alveoli collapse between breaths (atelectasis).