Geometric Hysteresis of Alveolated Ductal Architecture

Abstract

Low Reynolds number airflow in the pulmonary acinus and aerosol particle kinetics therein are significantly conditioned by the nature of the tidal motion of alveolar duct geometry. At least two components of the ductal structure are known to exhibit stress-strain hysteresis: smooth muscle within the alveolar entrance rings, and surfactant at the air-tissue interface. We hypothesize that the geometric hysteresis of alveolar duct is largely determined by the interaction of the amount of smooth muscle & connective tissue in ductal rings, septal tissue properties, and surface tension-surface area characteristics of surfactant. To test this hypothesis, we have extended the well-known structural model of the alveolar duct by Wilson and Bachofen (J. Appl. Physiol. 52(4): 1064–1070, 1982) by adding realistic elastic and hysteretic properties of 1) the alveolar entrance ring, 2) septal tissue, and 3) surfactant. With realistic values for tissue and surface properties, we conclude that: 1) there is a significant, and underappreciated, amount of geometric hysteresis in alveolar ductal architecture; and 2) the contribution of smooth muscle and surfactant to geometric hysteresis are of opposite senses, tending toward cancellation. Quantitatively, the geometric hysteresis found experimentally by Miki et al. (J. Appl. Physiol. 75(4): 1630–1636, 1993) is consistent with little or no smooth muscle tone in anesthetized rabbits in control conditions, and with substantial smooth muscle activation following methacholine challenge. The observed local hysteretic boundary motion of the acinar duct would result in irreversible acinar flow fields, which might be important mechanistic contributors to aerosol mixing and deposition deep in the lung.

Nebulisation of synthetic lamellar lipids mitigates radiation-induced lung injury in a large animal model

“…Simple models of lung parenchyma at the acinar level have surface tension at the air-liquid interface on alveolar septa pulling the alveolar ducts radially outward, an effect counterbalanced by tissue forces (collagen, elastin and smooth muscle) in the alveolar entrance rings pulling the ducts radially inward 61 . The balance between these elements contributes to pressure–volume and geometric hysteresis during breathing 62 , and hence gas mixing and deposition in the pulmonary acinus. It is tempting to speculate that the increase in ASMA expression seen after radiation exposure is compensation for an increase in surface tension brought about by an impaired surfactant system.…”

The role of anisotropic expansion for pulmonary acinar aerosol deposition

“…1a) are characterized by L ∝ L 0 , where L represents a time-dependent vector between a reference point and any location on the domain surface, and L 0 is its initial length at the onset of inhalation ( t = 0). First, we mimic the “cup to saucer” motion advanced by structural models of alveolar duct displacement (Denny and Schroter, 1997; Kojic et al, 2011); here, acinar motion is modeled by a hyperbolic-like radial expansion, where R∝boldR0−1/2 and R represents any radial vector orthogonal to the ductal centerline (Fig. 1b).…”

“…Such isotropic distensions are widely acknowledged to capture the principal mode of lung motion at a macroscopic whole-lung level (Ardila et al, 1974; Gil et al, 1979). Yet, it is has long been established that local anisotropy at the acinar scale lies at the origin of complex and heterogeneous deformations leading for example to geometrical hysteresis (Gil and Weibel, 1972; Kojic et al, 2011; Mead et al, 1957), a process observed amongst other in surface-to-volume loops during lung inflation–deflation maneuver (Miki et al, 1993).…”

Particle Transport and Deposition: Basic Physics of Particle Kinetics

“…The tips of the alveolar septa form the entrance rings of the alveolar duct and are rich with contractile elements 76,324 . This morphological arrangement might be strategically important in keeping the optimal shape and size of the alveolus in response to biological demand 104,187,327 . The preferential deposition of particles on the alveolar entrance rings might tip the balance of biological homeostasis (e.g., equilibrium of the gas-liquid interface).…”