Flow and Particle Dispersion in a Pulmonary Alveolus—Part I: Velocity Measurements and Convective Particle Transport

Abstract

The alveoli are the smallest units of the lung that participate in gas exchange. Although gas transport is governed primarily by diffusion due to the small length scales associated with the acinar region (approximately 500 microm), the transport and deposition of inhaled aerosol particles are influenced by convective airflow patterns. Therefore, understanding alveolar fluid flow and mixing is a necessary first step toward predicting aerosol transport and deposition in the human acinar region. In this study, flow patterns and particle transport have been measured using a simplified in-vitro alveolar model consisting of a single alveolus located on a bronchiole. The model comprises a transparent elastic 5/6 spherical cap (representing the alveolus) mounted over a circular hole on the side of a rigid circular tube (representing the bronchiole). The alveolus is capable of expanding and contracting in phase with the oscillatory flow through the tube. Realistic breathing conditions were achieved by exercising the model at physiologically relevant Reynolds and Womersley numbers. Particle image velocimetry was used to measure the resulting flow patterns in the alveolus. Data were acquired for five cases obtained as combinations of the alveolar-wall motion (nondeforming/oscillating) and the bronchiole flow (none/steady/oscillating). Detailed vector maps at discrete points within a given cycle revealed flow patterns, and transport and mixing of bronchiole fluid into the alveolar cavity. The time-dependent velocity vector fields were integrated over multiple cycles to estimate particle transport into the alveolar cavity and deposition on the alveolar wall. The key outcome of the study is that alveolar-wall motion enhances mixing between the bronchiole and the alveolar fluid. Particle transport and deposition into the alveolar cavity are maximized when the alveolar wall oscillates in tandem with the bronchiole fluid, which is the operating case in the human lung.

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

“…This finding led to the concept of chaotic mixing (i.e., flow-induced mixing), which may occur in the acinus [4,11]. Recently, numerous experimental [12][13][14][15][16] and numerical studies [17][18][19][20][21][22] of acinar flows have appeared in the literature, and many of these have confirmed the presence of recirculating alveolar flow. The occurrence of chaotic, or convective, mixing in the acinus is particularly interesting because no flow-induced mixing was assumed to occur in the classical descriptions of acinar flows [23].…”

Maximal efficiency of convective mixing occurs in mid acinus: A 3D-numerical analysis by an Eulerian approach

“…Following this work, several more numerical and experimental studies (Chhabra & Prasad, 2010;Haber et al, 2000;Henry et al, 2002;Kumar et al, 2009;Tsuda et al, 1999Tsuda et al, , 2008 have confirmed that the flow in the alveolar region is often kinematically irreversible and chaotic -more specifically, that a fluid particle entering an alveolus during inhalation would not follow the same pathway during exhalation. This flow irreversibility can also affect aerosol particles Haber et al, 2003;Tsuda et al, 2008) because fine particle transport in the alveoli includes a convective component in addition to a diffusive component.…”

“…In spite of a limited number of in vitro studies conducted to measure velocity flow fields in an expanding/contracting alveolus (Chhabra & Prasad, 2010;Tippe & Tsuda, 2000), these studies have clearly demonstrated the relationship existing between recirculation eddies developing inside the alveolus and the breathing conditions through the role of the Q A =Q D ratio. The agreement classically found between numerical and experimental approaches (Berg et al, 2010;Karl et al, 2004;Ma et al, 2009) is a confirmation of the validity and interest of the present approach.…”

“…This flow irreversibility can also affect aerosol particles Haber et al, 2003;Tsuda et al, 2008) because fine particle transport in the alveoli includes a convective component in addition to a diffusive component. In this regard it has been shown that particle deposition in alveoli is a function of residence time ) that in turn is influenced by the irreversible flow effects (Chhabra & Prasad, 2010;Darquenne et al, 2009;Lee & Lee, 2003;Tippe & Tsuda, 2000). However, quantifying the amount of mixing often requires computing the trajectories of a large number of particles, large enough to explore the various chaotic evolutions of a fluid volume initially located in the duct.…”

Airflow and Particle Deposition in Acinar Models with Interalveolar Septal Walls and Different Alveolar Numbers

“…Kumar et al [ 20 ] simulated airflow in acinar models with honeycomb structures and reported recirculation inside the alveoli induced by oscillatory wall motions. Talaat and Xi [ 15 ] numerically investigated aerosol deposition in a single terminal alveolus with rhythmical oscillations and found significantly different particle dynamics in comparison to that in alveolated ducts or respiratory bronchioles [ 12 , 21 , 32 , 33 ]. Particles move back and forth driven by the oscillating walls of the terminal alveolus and form multifolding trajectories [ 15 ]; by contrast, particles in an alveolated duct or respiratory bronchiole geometry remain suspended in the alveolus for several breathing cycles, rotating clockwise during exhalation and counterclockwise during inhalation [ 12 , 21 ].…”