Fig. 4.

Concepts of structural and functional opening and collapse of individual alveoli in injured lungs. a The concept of structural opening and collapse hypothesizes a complete anatomical collapse of the alveolus during end expiration with a subsequent re-opening of the collapsed alveolus during end inspiration. b The concept of functional collapse hypothesizes fluid shifts into and out of the alveoli in a tidal fashion, causing high shear stress which damages the alveolar epithelium and impairs alveolar function