GAS-EXCHANGE IN THE AIRWAYS

The primary function of the lungs is to exchange the respiratory gases , O-2 and CO2, between the atmosphere and the blood. Our overall under standing of the lungs as a gas-exchanging organ has improved considera bly over the past four decades. We now know that the dynamics of gas e xchange depend on the blood solubility (beta(b), ml gas ml blood(-1) a tm(-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). O-2 (beta(b) = 0.7) and CO 2 (beta(b) = 3.0) exchange primarily in the alveolar region of the lun g and their exchange is limited by the rate of ventilation and perfusi on. In contrast, highly soluble gases (beta(b) > 100) are likely to ex change primarily in the airways of the lung. We have used exhaled etha nol (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 m athematical model to analyze the dynamics of airway gas exchange. We m ake the following conclusions: (1) ethanol exchanges entirely within t he airways, and (2) the magnitude of perfusion- and diffusion-related resistance to airway gas exchange is the same.