Conducting Zone

Conducting airways do not contribute to the gas exchange and can be considered to be merely a conduit between the external environment and the respiratory zone (vide infra). The volume of air accommodated by the conducting airways represents the anatomic dead space and is air not directly available for gas exchange. Aside from serving as a conduit to the respiratory zone, the conducting airways perform two other functions: gas buffering and humidification.

The dead space (the volume of airway not involved indirectly in gas exchange) confers a buffering capacity on the airways, in that, for each breath, air taken in from the external environment or alveolar air must mix with dead-space air. This process, although decreasing the efficiency with which oxygen is delivered to and carbon dioxide is removed from the alveolar space (e.g., dead-space oxygen concentration will be determined by the oxygen concentration of air inhaled from the external environment and of that in the alveolar space that was shunted into the dead space in the previous breath), serves to even out the alveolar gas concentrations by preventing dramatic swings in alveolar gas concentrations that would occur if the alveolar air were exchanged completely during each breath. It should be recognized that the volume of the dead space is not insignificant. For example, in a normal tidal breath of 500-600 mL, 150 mL represents dead-space volume.

Inhaled air is humidified in the conducting airways through exposure to fluids lining these airways, a process that results in the delivery to the alveoli of air that is in isotonic equilibrium of 99.5% or greater relative humidity at body temperature.

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