Airway Humidification

Abstract

The alveolar air is fully saturated with water vapor at core body temperature while ambient air is cooler and contains less water. This gradient in heat and water vapor pressure is maintained along the nose and upper airways. They function as a counter current heat and moisture exchanger. The inspired air gains heat and water vapor from the upper airway lining which is partly recovered when the expired gas looses heat and water condenses back to the airway surface. This recovery occurs because the upper airway temperature remains below core body temperature during expiration. Breathing is associated with a net heat and water loss because the expired air temperature is higher than ambient temperature under normal circumstances. The losses must be replenished by the airway epithelium which in turn is supplied by the bronchial circulation. It is unknown under which circumstances the capacity of the airway lining to humidify cold and dry gas becomes overcharged. This capacity is likely different in health and disease. Water transport through the mucosa into the aqueous layer of the airway lining is possibly rate limiting.