Airway Surface Liquid and Respiratory Mucus - Tissue Functioning and Remodeling in the Circulatory and Ventilatory Systems - page 768

12.5 Mucus Properties

Biophysical properties of airway secretions include rheology, cohesivity that results
from molecular attraction and depends on polymer length and density, and surface
properties.
Rheological properties deal with deformation and energy storage when a stress
is applied and release of stored energy when the stress is removed. Restitution of
stored energy explains mucus clearability. The respiratory mucus is characterized by
a viscoelastic behavior. Mucus actually possess pseudoplasticity , elastothixotropy ,
spinnability (capacity to be drawn out into long threads [Spinnbarkeit]), and
adhesiveness [ 1529 ].
Surface interactions of the airway surface liquid with the respiratory epithelium
on the one hand and airflow on the other comprise wettability (interfacial spreading
capacity measured by the contact angle), adhesiveness (interfacial tension quantified
by adhesivity), and tenacity (product of adhesive work and cohesivity). Contact and
rheological properties determine mucus transport function. The latter also depends
on mucosal permeability to particles.

12.5.1 Mucus Permeability

Airway surface liquid operates as a protective barrier at mucosal surfaces that traps
and removes foreign particles such as bacteria and toxins (size

1 mm) as well
as viruses and environmental small particles (size 100-500 nm), but allows flux of
nutrients, selected gases, ions, antibodies, and many proteins as well as cells of
the mucosal immune system. It is impermeable to large particles, the permeability
threshold depending on strength of association of mucin fibers that determines the
typical mucin mesh size. Selective permeability and dynamic viscoelastic behavior
of mucus are also controlled by mucin environmental condition (lipids, salts, cells,
etc.). Mucus behavior is governed by viscous drag of interstitial fluids moving
within the mucin mesh and elastic recoil from mucin fibers.
Mucus is characterized by heterogeneity of the pore network of the viscoelastic
matrix made of highly branched and strongly negatively charged mucus glycopro-
teins that are entangled with other mucosal constituents.
Diffusion of amine-modified and carboxylated polystyrene particles 19 (size 100-
500 nm) has been investigated in sputum of mucoviscidosis patients by particle
tracking with good spatial and temporal resolution (5 nm and 33 ms) [ 1561 ]. Small
(size

∼

200 nm), neutrally charged particles travel more quickly inside mucus than
large, charged particles. Particle transport rate depends on the particle size range, as

<

19 Amine-modified and carboxylated polystyrene particles have neutral and negative surface
charges at physiological pH, respectively.