The respiratory system is a highly balanced mechanism reliant on humidity. To provide therapies that maintain optimal lung function it is necessary to understand the physiological balance of humidity in the airway.
TWO MAIN LUNG FUNCTIONS
Gas Exchange
Air-flow to the alveoli is necessary for gas exchange. The natural heating and humidifying functions of the airway assist with maintaining clear and patent airways by promoting mucociliary clearance and reducing bronchoconstriction associated with airway cooling.
Airway Defense
Primary mechanical defense mechanisms are sneezing, coughing, gagging and the use of natural filters, i.e. nasal hairs. The second line of defense is the mucociliary transport system which traps and neutralizes inhaled contaminants (in mucus) and transports them up and out of the airway, keeping the lung free from infection-causing pathogens. This critical defense system is very sensitive to humidity.
HUMIDITY MAINTAINS EFFICIENT GAS EXCHANGE AND PRESERVES AIRWAY DEFENSE
As air travels down the airway, heat and moisture is drawn from the airway mucosa to the point where the gas reaches 37 °C, 44 mg/L at the carina. The majority of this conditioning is carried out in the nasopharynx. It is important for the airway mucosa to retain a balance of heat and moisture to maintain a fully functioning mucociliary transport system and an efficient line of defense. This in turn plays an important role in efficient gas exchange by maintaining clear and open airways with effective mucus clearance.
A HEALTHY MUCOCILIARY TRANSPORT SYSTEM
The mucociliary transport system is comprised of three layers: mucus, the aqueous layer and ciliated epithelium. These layers all contribute heat and moisture to ensure a finely tuned mucociliary transport system. Millions of cilia lining the airway (around 200 individual cilia per cell) beat in the aqueous layer at up to 15 times per second. The beating cilia clear mucus and contaminants out of the airways. The clearance speed relies on the cilia beat frequency and quality of the mucus. Both are dependent on the body’s ability to replenish moisture to all three layers.
1. Williams R, Rankin N, Smith T, Galler D, Seakins P. Relationship between the humidity and temperature of inspired gas and the function of the airway mucosa. Crit Care Med 1996; 24(11):1920-9.