elastic recoil
Elastic recoil is the lung's built-in urge to shrink back toward a smaller size after being stretched, like a rubber band snapping back once you stop pulling. Inflate the lung, then let go, and it pulls itself in — that inward pull is recoil.
Two things create this springiness. One is the elastin and collagen fibres woven through lung tissue, which resist stretch. The other, surprisingly larger, is surface tension at the moist alveolar lining, where water molecules pull inward; surfactant tames this so the recoil does not become overwhelming.
Recoil does real, useful work. It is the engine of passive expiration — the stored elastic energy of inspiration drives air out without muscular effort. It also tugs outward on the small airways, holding them open during exhalation so air can escape freely.
Losing recoil is the central problem in emphysema: when the elastic framework is destroyed, the lung becomes floppy, exhalation slows, and airways collapse and trap air. Too much recoil — as in fibrosis — does the opposite, making the lung stiff and hard to inflate.
Pursed-lip breathing helps emphysema patients by adding back-pressure that substitutes for lost recoil, holding the floppy airways open long enough to exhale more completely.
Pursed-lip breathing compensates for lost recoil.
Elastic recoil does double duty: it empties the lung and it splints the small airways open. When emphysema destroys it, both jobs fail at once — air comes out slowly and the airways collapse on themselves.