Airway Surface Liquid and Respiratory Mucus - Tissue Functioning and Remodeling in the Circulatory and Ventilatory Systems - page 778

Table 12.6. Factors that stimulate or inhibit mucociliary clearance.

Factor class Increase Decrease

Biochemical Adenosine Leukotriene-C4

factors cAMP-PKA signaling

PLC-Ca 2 + axis
Nitric oxide
Adrenergic agonists
Cholinergic agonists
Histamine
AT P
Amiloride
Hyperosmolar solution Atropine

Environmental Acute exposure to Chronic exposure to

factors sulfur dioxide, sulfur dioxide,

cigarette smoke, cigarette smoke,

ozone ozone,

NO 2

Pathological Pseudohypoaldosteronism Cystic fibrosis

factors Chronic cough Primary ciliary dyskinesia

Chronic bronchitis
Asthma
Respiratory infections

Transport speed depends on air flow and airway bore, as well as mucus layer
thickness, rheology, and production rate [ 1587 , 1588 ]. The air flow rate required
for upward transport in vertical tubes of a quasi-uniform liquid layer of rheological
properties comparable to human sputum is in the Reynolds number range of 142
to 1132 and 708 to 2830 whether the tube internal diameter is equal to 5 mm or
1 cm, depending on the liquid types. The mean mucus layer thickness range from
0.2 to 0.5 mm and from 0.8 to 1.4 mm whether the internal diameter is equal to 5 mm
or 1 cm. The mean mucus layer thickness diminishes with increasing flow rate and
decreasing mucus viscosity. The advancing speed of the leading edge of mucus layer
in vertical tube ranges from 1.1 to 3.1 cm/min with a mucus feed rate of 0.5 ml/min
at air flow rates of 0.33 to 1.17 l/s. The mucus speed increases almost proportionally
with elevating mucus feed rate. The mucus speed rises with increasing air flow rates
and decreases with increasing mucus viscosity. The transport speed in a horizontal
tube is 5 to 60% faster than in a vertical tube.
Propulsion of mucus is assisted by coughing to expel large inhaled materials and
chloride pumps to osmotically hydrate mucosal secretions. When the respiratory
mucus interacts with air flow during coughing, micron-sized droplets form and
are expelled to the environment. The droplets can hence possibly participate in
propagation of respiratory infections.