Morphology of mucus films in lung airways: secretion and ciliary evacuation

TL;DR

This study uses simulations to explore how mucus films in lung airways behave under different secretion rates, revealing conditions that lead to uniform coatings, unduloid humps, or mucus plugs, which are crucial for understanding respiratory health.

Contribution

The paper introduces a reduced-order thin-film model to simulate mucus film dynamics in airways, highlighting the effects of secretion rates and ciliary transport on film morphology.

Findings

Low secretion maintains a uniform mucus film.

Increased secretion causes unduloid-shaped humps.

High secretion leads to mucus plug formation.

Abstract

Lung airways are lined by a film of mucus which protects the epithelium from inhaled particles. To maintain a uniform coating, the mucus that is secreted into airways must be distributed into a film by wall-attached cilia, which constantly convey mucus along the airway. However, the film's natural tendency is to accumulate into humps and plugs, due to the Rayleigh-Plateau instability. To understand the behaviour of the film amidst these competing factors, we perform simulations of an idealized tubular airway using a reduced-order thin-film model. The axial boundaries are nonperiodic, allowing for cilia-driven inflow and outflow; a tangential velocity along the tubular wall models ciliary transport, while a localized source at the wall accounts for secretion. On increasing the mucus input rate, we find three distinct film morphologies: (i) uniform flat films; (ii) nonuniform films that are composed of travelling unduloid-shaped humps, separated by mucus-depleted zones; and (iii) films that form an occluding plug. The flat-film regime, absent in closed periodic domains, emerges as a consequence of the convective nature of the Rayleigh-Plateau instability in the presence of ciliary transport. Higher secretion rates increase the mean film-thickness and induce a convective-to-absolute transition, which manifests in the appearance of travelling humps. In the plug-forming regime, capillary forces dominate and drive the incoming mucus into a single hump, which resists ciliary translation and remains near the mucus source. Our results show how low-level baseline secretion sustains a protective, uniform mucus film, and how hypersecretion -- stimulated, for example, by inhaled allergens -- produces mucus plugs.