Geometric Hysteresis of Alveolated Duct Architecture

The alveolar duct changes its geometry during tidal breathing. At least two components of the ductal structure are known to exhibit hysteresis: smooth muscle within the alveolar entrance rings, and surfactant at the air‐tissue interface. We hypothesize that the overall mechanical behavior of lung (including both p‐V and geometric hysteresis) is largely determined by the interaction of: the amount of smooth muscle in ductal rings, septal tissue properties, and surface area‐surface tension characteristics of surfactant. To test this hypothesis, we have extended our previous duct model (FASEB J. 19(4):A640, 2005) by adding 1) nonlinear elasticity and smooth muscle hysteresis to model the behavior of the entrance ring, and 2) a biaxial nonlinear elasticity to model the behavior of the septal walls. Surface area‐surface tension hysteresis is also included. We found good agreement between our computed p‐V relationships and experimental observations. Results showed that: 1) there is significant, and underappreciated, amount of geometric hysterisis in alveolar ductal architecture; 2) as expected, smooth muscle and surfactant hysteresis both contributed to p‐V hysteresis in the same sense (i.e. dissipation); but 3) the contribution of smooth muscle and surfactant to geometric hysteresis are of opposite senses, tending toward cancellation. Finally, we computed the radial distribution of the remaining geometric hysteresis, which in turn leads to the first semiquantitative estimates of kinematic irreversibility of acinar airflow and alveolar ventilation. Supported by NIH HL54885, HL70542, and HL74022.