Airway Surface Liquid and Respiratory Mucus - Tissue Functioning and Remodeling in the Circulatory and Ventilatory Systems - page 743

Chapter 12
Airway Surface Liquid and Respiratory Mucus
Mucosa is a lining involved in absorption and secretion of mucus, especially, among
other materials. Mucosal surface comprises a wet epithelium, a structural barrier,
covered by a protective mucous barrier. The mucus layer in airways is thin and
mobile.
Human conducting airways are mostly lined with a pseudostratified, secretory,
and ciliated epithelium that comprises 3 major cell types — ciliated, secretory, and
basal cells — with submucosal glands and cartilaginous elements in large bronchi
(Vol. 1 - Chap. 3. Cells of the Ventilatory Tract).
The lung resists to damages caused by the body's outside environment following
a permanent exposure to pathogens and more or less toxic chemical pollutants in
inhaled air. Airway mucus represents the first line of defense. Inhaled particles
(allergens, dust, microorganisms, and cellular debris) trapped in the mucus gel are
removed from the respiratory tract by the mucociliary clearance. In fact, airway
mucus traps inhaled toxins and transfers them out of the airways not only using
mucus transport that results from ciliary beats, but also cough. Mucus cleans
airways, as it flows from either the tracheobronchial tree or upper airways toward
the pharynx, where it is swallowed (or expectorated).
The airway surface liquid consists of 2 components: (1) a gel-like mucus layer
that traps inhaled particles and transports them out of airways by a cilia-generated
motion and (2) a periciliary liquid layer that supports ciliary beating, hydration of
airway mucus, and lubrication of the wetted epithelial surface. Cilia of respiratory
epithelia beat in a coordinated manner within a periciliary fluid underneath a layer
of mucus gel and transiently impact the mucus layer during the beating cycle,
thereby avoiding an oscillatory motion. The luminal gel comprises water and heavily
glycosylated mucins.
Pulmonary epithelial cells provide a mechanical barrier to microbial entry and
participate in innate defense. They sense the presence of microbes and then augment
their barrier function to resist to microbial penetration, signal to leukocytes, release