New insights into the mechanisms controlling the bronchial mucus balance

Abstract

In this work, we aim to analyze and compare the mechanisms controlling the volume of mucus in the bronchial region of the lungs of a healthy human adult, at rest and in usual atmospheric conditions. This analysis is based on a balance equation for the mucus in an airway, completed by a computational tool aiming at characterizing the evaporation, during respiration, of the water contained in the bronchial mucus. An idealized representation of the lungs, based on Weibel's morphometric model, is used. The results indicate that the mechanisms controlling the volume of mucus in an airway depend on the localization of the airway in the bronchial region of the lungs. In the proximal generations, the volume of mucus in an airway is mainly controlled by the evaporation of the water it contains and the replenishment, with water, of the mucus layer by epithelial cells or the submucosal glands. Nevertheless, cilia beating in this part of the bronchial region remains of fundamental importance to transport the mucus and hence to eliminate dust and pathogens trapped in it. On the other hand, in the distal generations of the bronchial region, the volume of mucus in an airway is mainly controlled by the mucociliary transport and by the absorption of liquid by the epithelium. This absorption is a consequence of the mucus displacement by the cilia along generations with an interface between the epithelium and the airway surface layer of decreasing area. The numerical results obtained are in good agreement with previously published experimental data, thus validating our approach. We also briefly discuss how our results can improve the understanding and, possibly, the treatment of pulmonary diseases.

Fig 1. Three levels of details of the human bronchial region.

a) Picture of a lung cast showing the human bronchial region. b) Schematic transverse view of an airway, with the different layers surrounding the lumen of the airway. c) The epithelial cilia penetrate in the ASL and generate the displacement of the mucus layer.

Fig 2. Scheme of an airway in generation i and a bifurcation.

Several parameters related to the balance of the mucus in the airway are represented on this figure. The picture is not to scale.

Fig 3. Results obtained with the computational tool.

Left: calculated value of E_i/S_i as a function of the generation index. Right: calculated average temperature of the air on a flow cross-section as a function of the distance from the top of the trachea, at the end of the inspiration (circles) and the expiration (squares). These results have been obtained with the computational tool, using tⁱⁿ = 2 s, f = 0.25 s⁻¹, Qⁱⁿ = 250 ml s⁻¹, Tⁱⁿ = 30°C and RHⁱⁿ = 0.80.

Fig 4. Magnitude of the various mechanisms controlling the bronchial mucus balance.

The results were obtained with the data given in Table 1. Left figures: Tⁱⁿ = 30°C and RHⁱⁿ = 0.80, right figures: Tⁱⁿ = 34°C and RHⁱⁿ = 0.95. Top figures: δ_μ1 = 10 μm, v_μ1 = 5 mm min⁻¹, δ_μ16 = 2 μm and v_μ16 = 0.5 mm min⁻¹. Bottom figures: δ_μ1 = 30 μm, v_μ1 = 5 mm min⁻¹, δ_μ16 = 3 μm and v_μ16 = 0.5 mm min⁻¹. Squares-continuous lines: ΔM_i, diamonds-dashed lines: E_i, circles-dotted lines: B_i.

Fig 5. Schematic diagram of an airway, during inspiration.

Several notations used in the model are presented on this figure. The picture is not to scale.