Viscoelastic and Rheological Properties of Mucus

Disease conditions such as CF, chronic obstructive pulmonary disease (COPD), and emphysema generally result in an increase in the viscoelasticity of mucus, owing in part to reduced liquid content and an increased percentage of highly branched glycoproteins [145]. The viscoelasticity of mucus is closely regulated (in the normal lung) to provide the proper elasticity to resist gravitational flow while minimizing viscosity to attain rapid mucociliary transport [120]. Mucociliary transport is responsible for clearing matter from the upper conducting airways, via the larynx, to the gut [146]. For mucociliary transport to work efficiently, the viscosity of mucus should be sufficient to support a load without hindering the ability of cilia to move it toward the mouth. Mucus velocity has been estimated in vitro by measuring the velocity of tracer particles on depleted frog palates, incised tracheal tissues, or intact airways using fiber-optic bronchoscopy [147]. The rate of mucociliary transport in humans is 5 -10 cm/min [120]. This rate may be affected, however, by diseases or inhalation of mucolytic agents.

Changes in the viscoelasticity of mucus gels alter the mucus clearance rate and, hence, particle transport efficiency. An increase in viscosity [148], or a decrease in elasticity [132,149] of mucus gels leads to slower mucociliary clearance rates, allowing particles a longer time to penetrate mucus.

However, mucus permeability is reduced as viscosity increases, owing to the increased viscous drag and steric obstruction from multiple mucus-particle interactions [120,145,150]. One might expect that an increase in elasticity of mucus gels would lead to caging of particles moving through the mesh [151], thereby reducing particle permeability. However, Sanders and coworkers related variations in the elastic modulus (G0) of mucus samples to particle transport and found that the percentage of particles (124-270 nm) transported through mucus increased with increasing elasticity (G0 > 100 Pa) [144] (Fig. 3). This unexpected finding was explained by a possible increase in the heterogeneity of the mucus mesh, allowing larger pores for particle transport as elasticity increased.

The rheological properties of mucus forming important to particle transport through the mucosal barrier, including viscosity and elasticity, are dependent on the shear rate along the tracheobronchial tree. Shear is imposed by ciliary motion and coughing [152]. Mucus acts as a thixotropic gel, meaning its viscosity increases with increasing shear rate initially and then begins to decrease [138,153]. The critical shear rate for respiratory mucus, at which viscosity begins to decrease, is approximately 1 sec"1 [153]. By modeling the tracheobronchial regions of the lungs as cylindrical tubes, average shear rates were estimated as 0.91 sec"1 (large bronchi), 0.78 sec"1 (medium bronchi), and 0.25 sec"1 (small bronchi) [152]. Thus, mucus acts elastically at physiological shear rates in

Figure 3 Percentages of nanospheres that diffused through 220-|mm-thick layers of CF and COPD (gray circle) sputum and into the acceptor compartment of the diffusion chamber system after 150 min as a function of the elastic moduli of the sputum samples (n = 4 for each data point). (Reprinted form Ref. 144. Courtesy of the American Journal of Respiratory and Critical Care Medicine.)

elastic modulus (Pa)

Figure 3 Percentages of nanospheres that diffused through 220-|mm-thick layers of CF and COPD (gray circle) sputum and into the acceptor compartment of the diffusion chamber system after 150 min as a function of the elastic moduli of the sputum samples (n = 4 for each data point). (Reprinted form Ref. 144. Courtesy of the American Journal of Respiratory and Critical Care Medicine.)

the upper airways, thereby maintaining the shape of mucus fibers and the structure of the mucus fiber networks [152].

Coping with Asthma

Coping with Asthma

If you suffer with asthma, you will no doubt be familiar with the uncomfortable sensations as your bronchial tubes begin to narrow and your muscles around them start to tighten. A sticky mucus known as phlegm begins to produce and increase within your bronchial tubes and you begin to wheeze, cough and struggle to breathe.

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