Gas Transport Processes In Human Airways

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Abstract
Numerous mathematical models describe the behavior of inhaled gases of health effects concerns. Such models are often developed for particular applications and have commensurate limitations (e.g., applicability to water-soluble or -insoluble gases) based upon the assumptions used in their respective derivations. The various models have certain restrictions in common; for instance, they assume airways of the lung to be smooth-walled tubes with fixed flow conditions. But, as documented herein, airways of the lung contain pronounced surface irregularities, including natural structural features like cartilaginous rings in the upper bronchi, and various airway lengths insufficient for flow development. Therefore, we have focused upon the behavior of inhaled gases within human conducting airways while accounting for effects of localized features of morphology and developing flow conditions. The role of respiratory intensity (i.e., ventilatory parameters) was also examined. The new model has been developed to quantitatively describe convective gas diffusion in a lung environment that is realistic from perspectives of biology and physics. The results include the following observations. (1) The Schmidt number is important for the characterization of inhaled gases. (2) Human activity levels have considerable effects on gas diffusion efficiencies within the lung; for example, the difference between diffusion efficiencies during sedentary and light activity conditions is about 100% for a gas Schmidt number of 1. (3) Airway surface irregularities have prominent effects on gas transport. For instance, diffusion efficiencies can be enhanced by up to 35% (relative to smooth-walled tubes) due to the presence of cartilaginous rings in the large airways of the tracheobronchial tree.