Fig. 4.

Geometric hysteresis within the alveolar duct. (a) Case without surfactant. Geometric hystersesis is due to hysteresis of muscle constituent within the alveolar entrance ring and it increases with increasing m. The S-V hysteresis loop has a clockwise direction for one breathing cycle. (b) Case with surfactant. The S-V hysteresis loop due to surfactant has counterclockwise direction (condition with no muscle constituent, m = 0). Since S-V hysteresis loops corresponding to muscle and surfactant have the opposite senses, the resulting hysteresis is smaller when m = 0.5 with respect to m = 0 and the loop changes the direction for m = 1.