The Simultaneous Role of an Alveolus as Flow Mixer and Flow Feeder for the Deposition of Inhaled Submicron Particles

Abstract

In an effort to understand the fate of inhaled submicron particles in the small sacs, or alveoli, comprising the gas-exchange region of the lung, we calculated the flow in threedimensional (3D) rhythmically expanding models of alveolated ducts. Since convection toward the alveolar walls is a precursor to particle deposition, it was the goal of this paper to investigate the streamline maps' dependence upon alveoli location along the acinar tree. On the alveolar midplane, the recirculating flow pattern exhibited closed streamlines with a stagnation saddle point. Off the midplane we found no closed streamlines but nested, funnel-like, spiral, structures (reminiscent of Russian nesting dolls) that were directed towards the expanding walls in inspiration, and away from the contracting walls in expiration. These nested, funnel-like, structures were surrounded by air that flowed into the cavity from the central channel over inspiration and flowed from the cavity to the central channel over expiration. We also found that fluid particle tracks exhibited similar nested funnel-like spiral structures. We conclude that these unique alveolar flow structures may be of importance in enhancing deposition. In addition, due to inertia, the nested, funnel-like, structures change shape and position slightly during a breathing cycle, resulting in flow mixing. Also, each inspiration feeds a fresh supply of particleladen air from the central channel to the region surrounding the mixing region. Thus, this combination of flow mixer and flow feeder makes each individual alveolus an effective mixing unit, which is likely to play an important role in determining the overall efficiency of convective mixing in the acinus.

Role of Alveolar Topology on Acinar Flows and Convective Mixing

“…Flow patterns in medial generations are characterized by a gradual transition from recirculating to radial flows ( Fig. 2(b)), with a pronounced shift in the location of the vortex center toward the proximal side of the alveolus [25,38,46,47]. Note that for all values of QAIQD, alveolar velocities inside the cavities are orders of magnitude smaller compared to the ductal flow (see logarithmic scale in Fig.…”

“…In addition to Re and a, alveolar flow patterns are known to be governed by the ratio of alveolar (g^) to ductal flow rate (ÖD) [25,38,46,52]; a dimensionless parameter that depends on the volume of the subacinus fed distally to the considered duct and, hence, correlates with acinar generation [10]. As long as ducts and alveoli follow the same self-similar oscillatory displacement function (see below), the ratio QA/QD is purely geometric and does not depend on breathing frequency or inhaled volume [10,32].…”

“…With tangible progress in imaging modalities, including micro-computed tomography (CT) and radiation-based x-ray tomographic microscopy (XTM), three-dimensional (3D) anatomical reconstructions of pulmonary acini are gaining traction for studies on lung development [19], acinar morphometry [20][21][22], and acinar mechanics [17]. However, their implementation for computational fluid dynamics is still limited to simulations in terminal alveolar sacs or a few alveoli surrounding a single duct only [23][24][25], Hence, much of our understanding of alveolar flows and particle transport results from studies in idealized acinar geometries.…”

“…Namely, the alveolar topology defines the location of the separation surface and the ensuing vortex [33], underlining the importance of the relative position of the alveolar mouth opening [13,45]. The role of alveolar topology is further emphasized under rhythmic motion of the alveolar septum during breathing [13,46], where cyclic wall motion guarantees convective exchange between alveolar and ductal flows [25,38,40]. Furthermore, wall displacements deliver a mechanism by which inhaled particles with little intrinsic motion can still enter the alveolar airspace where convective flow mixing may operate [47^9].…”

Submicron particle deposition in pulmonary alveoli during cyclic breathing

“…They reported 33% and 75% deposition e ciency, respectively, for 1 and 3 m aerosols under normal gravity condition. Henry et al [12] reported ow mixing mechanism in rhythmically expanding idealized spherical models of alveolus for submicron particles. In these studies, a sinusoidal function of time was used for expansion and contraction of alveolus, where it was assumed that the model deformed isothropically.…”