Gas Exchange in the Airways

Abstract

ABSTRACT

The primary function of the lungs is to exchange the respiratory gases, O₂ and CO₂, between the atmosphere and the blood. Our overall understanding of the lungs as a gas-exchanging organ has improved considerably over the past four decades. We now know that the dynamics of gas exchange depend on the blood solubility (β_b, ml gas ml blood⁻¹ arm⁻¹) of the gas. While the major focus of research has rightly been on the respiratory gases, the lungs exchange a wide spectrum of gases ranging from very low solubility gases such as SF₆ or helium (β_b = 0.01) to water vapor (β_b = 20,000). O₂ (β_b = 0.7) and CO₂ (β_b = 3.0) exchange primarily in the alveolar region of the lung and their exchange is limited by the rate of ventilation and perfusion. In contrast, highly soluble gases (β_b > 100) are likely to exchange primarily in the airways of the lung. We have used exhaled ethanol (β_b = 1756) profiles for humans, steady-state exchange of six inert gases (0.01 <β_b< 300) in an in situ dog trachea, and a mathematical model to analyze the dynamics of airway gas exchange. We make the following conclusions: (1) ethanol exchanges entirely within the airways, and (2) the magnitude of perfusion- and diffusion-related resistance to airway gas exchange is the same.