A 4-Dimensional Model of the Alveolar Structure

Abstract

Abstract:The alveolar structure, a space-filling branching duct system with alveolar openings, is one of the most complicated structures in the living body. Although its deformation during ventilation is the basic knowledge for lung physiology, there has been no consensus on it because of technical difficulties of dynamic 3-dimensional observation in vivo. It is known that the alveolar duct wall (primary septa) in the fetal lung is deformed so as to obtain the largest inner space and the widest surface area, and that the secondary septa grow just before birth and their free ridges form the alveolar entrance rings (mouths) containing abundant elastin fibers. We have constructed a 4-dimensional alveolar model according to this morphogenetic process, where the alveolar deformation is modeled by a combination of springs and hinges, corresponding to elastin fibers at alveolar mouths and junctions of alveolar septa, respectively. The model includes a hypothesis that alveolar mouths are closed at minimum volume and that closed alveoli are stabilized by the alveolar lining liquid film containing a surfactant. Morphometric characteristics of the model were consistent with previous reports. Furthermore, the model explained how the alveolar number and size could change during ventilation. Using in vivo microscopy, we validated our model by an analysis of the dynamic inflation and deflation of subpleural alveoli. Our model, including the alveolar mouth-closure hypothesis, can explain the origin of phase IV in a single breath nitrogen washout curve (closing volume) and mechanism of alveolar recruitment/derecruitment.

Lung volume recruitment maneuvers and respiratory system mechanics in mechanically ventilated mice

“…The findings in the present study therefore support the view that the improvement of estimates of respiratory system mechanics after large RMs is due to fundamental changes in quasi-static and dynamic lung compliance, as substantiated by the inflation PV curve and the values of R aw , G, and H, respectively. Though the mechanisms responsible for the increased lung compliance above P ao of 25 cmH 2 O are not clear, alveolar unfolding and surfactant redistribution have been proposed as possible explanations (Soutiere and Mitzner, 2004;Escolar and Escolar, 2004), while others suggest that alveolar mouths, previously closed by a surfactant-lined liquid film, open during recruitment of peripheral lung units at high transpulmonary pressures providing a new population of available alveoli (Scarpelli, 1998;Kitaoka et al, 2007;Namati et al, 2008 …”

Investigation of the Effects of Emphysema and Influenza on Alveolar Sacs Closure through CFD Simulation

“…Kitaoka et al [17] used a four-dimensional alveolar model, corresponding to elastin fibers at alveolar mouths and junctions of alveolar septa to simulate alveolar deformation. Hansen et al [18] studied different airway branching patterns and air spaces of a single human terminal bronchiole.…”

A Novel Interpretation of Closing Volume Based on Single-Breath Nitrogen Washout Curve Simulation

“…Recently, we have constructed a 4-D (3-D + time axis) alveolar model in which alveolar mouths are closed at their minimum volumes under the existence of pulmonary surfactant function [23]. Our model provides a uniform value of maximum tissue density at which the alveolar mouth is closed with trapped air inside.…”

“…Our model also reveals that the spatial disconnection sites during phase IV [6][7][8] is the alveolar mouth. The alveolar mouth closure is never imaginary, but is recorded with rapidly frozen histological sections in literature, though the original authors did not mention it [23,24]. Figure 8 is a reproduction of a figure in the paper by Young et al [24].…”