FIGURE 9.

Viscoelastic behavior of alveolar recruitment and derecruitment described using a spring and dashpot model. During inspiration the spring is rapidly stretched, whereas the dash in the pot moves more slowly as fluid squeezes around it (brown arrows), in response to the applied force (i.e., tidal volume – red bar). The effect of this viscoelastic behavior is continual alveolar recruitment as long as inspiration is held (open lung/time curve). Viscoelastic behavior during expiration shows a rapid collapse of the spring and a much slower collapse of the dash in the pot. This is shown in the open lung/time curve as continuous alveolar collapse or derecruitment over time. Viscoelastic alveolar opening and collapse suggests that the longer inspiration is held, the more alveoli are recruited, and the shorter the expiratory time, the fewer the alveoli that collapse.