Airway Surface Liquid and Respiratory Mucus - Tissue Functioning and Remodeling in the Circulatory and Ventilatory Systems - page 770

Table 12.4. Mucus rigidity measured by magnetic rheometry at low rotation speed (1 rad/s) and
clearance indices (Source: [ 1565 ]).

Rigidity factor Mucociliary

(log) clearability Cough Secretion rate

index index clearability (mg/min)

Baseline 2.25 ± 0.60 0.9 ± 0.1 1.3 ± 0.6 0.12 ± 0.12

Acetylcholine 1.58 ± 0.40 1.0 ± 0.1 1.8 ± 0.6 0.63 ± 0.49

Tasuldine 1.80 ± 0.64 1.0 ± 0.1 1.7 ± 0.7 0.11 ± 0.12

of respiratory mucus vary according to the site along the airway network. Mucus
heterogeneous microscopic domains contain entangled fibers and spaces filled by a
low-viscosity fluid. For a given sampling site, mucus rheology depends not only
on loading strength and application rate, but also constituent crosslinking and
entanglement.
Airway surface liquid is subjected to low and high stresses that correspond to
cilium beat and cough, respectively. An optimal rheology of the airway surface
liquid is required for the mucociliary clearance, as its deformation features deter-
mine efficiency of its coupling to cilia. Mucus with a low or high storage modulus
(i.e., small or large number of crosslinks) yields reduced transport speed. When
mucus becomes too thick (e.g., in severe bronchitis or cystic fibrosis), mucociliary
clearance decays and bacterial growth is favored.
Elasticity is the capacity to store a recoil force and restore energy, whereas
viscosity that measures resistance to deformation is associated with energy loss.
Elasticity of a mucus gel depends on density of crosslinks and time scale over
which crosslinks rearrange in response to applied stress. Mucus viscosity decays
with heightening stress or rate of strain.
Dynamic viscosity of normal mucus is s. 22 Current values of the dynamic

storage (elastic) modulus range 1 to 2 N/m 2 for efficient transport. The crosslinking
degree of mucus gel influences the storage modulus.
Glycosphingolipids increase mucus viscosity. Mucus viscosity is correlated with
sialomucin concentration. In patients with cystic fibrosis, cholesterol, glycosphin-
golipids, and sphingomyelin are positively correlated to the viscosity of expecto-
rated airway secretions, whereas phosphoglycerol is negatively correlated [ 1564 ].
Certain bronchosecretolytic agents decrease and increase the sialomucin and sul-
fomucin content, respectively, thereby attenuating mucus viscosity [ 1565 ](Ta-
ble 12.4 ).

∼ 1Pa ·

22 In normal, anesthetized dogs, mean values of elasticity and viscosity of tracheal mucus at 1
rad/s are about 30 Pa and 7.5 Pa · s , respectively, whereas steady-state viscosity is approximately
3 × 10 3 Pa · s[ 1563 ]. Mean viscosity as well as water content (98%) of airway liquid in normal
(0.6 ± 0.5 Pa.s) and cystic fibrosis (0.2 ± 0.1 Pa · s) fetal tracheal xenografts is similar, although
previous literature data report high viscosity values in cystic fibrosis (from 1 Pa · sto2kPa · s) [ 1560 ].
Hyperviscosity and dehydration in cystic fibrosis airway secretions result from inflammation and
infection.