Gas exchange in the airways.

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 (beta b, ml gas ml blood-1 atm-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 (beta b = 0.01) to water vapor (beta b = 20,000). O2 (beta b = 0.7) and CO2 (beta 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 (beta b > 100) are likely to exchange primarily in the airways of the lung. We have used exhaled ethanol (beta b = 1756) profiles for humans, steady-state exchange of six inert gases (0.01 < beta b < 300) in an in situ dog trachea, and a mathematical model to analyze the dynamics of airway gas exchange. We make the following conclusion: (1) ethanol exchanges entirely within the airways, and (2) the magnitude of perfusion- and diffusion-related resistance to airway gas exchange is the same.