Relative Importance of Diffusion and Chemical Reaction Rates in Determining Rate of Exchange of Gases in the Human Lung, With Special Reference to True Diffusing Capacity of Pulmonary Membrane and Volume of Blood in the Lung Capillaries

Abstract

An equation, i/D_m + i/θV_c = i/Dl, has been derived which relates the measured pulmonary diffusing capacity (Dl), the true diffusing capacity of the pulmonary membrane (Dm), the rate of uptake of CO by the red cells per mm Hg CO tension (θ) and the blood volume of the pulmonary capillary bed (V_c). By making measurements of Dl at different alveolar O₂ tensions, thereby causing to vary, this equation can be solved graphically for Dm and V_c, which are assumed to be independent of O₂ tension. Calculations of Dm and V_c were made utilizing a) values of θ previously obtained from the in vitro rates of CO uptake of suspensions of human red cells at 37°C and b) values of Dl in normal resting subjects at alveolar O₂ tensions from about 100 mm Hg to over 600 mm Hg measured by both steady state and breath holding CO techniques. Dm is about twice the value of Dl measured in subjects breathing air at sea level. V_c is about 75 ml in approximate agreement with the previously reported estimate of Roughton. Similar results were obtained using values of Dl at different alveolar O₂ tensions reported in the literature. This means that, in determining the rate of CO absorption in the lungs, the resistance of the red cell to the uptake of CO is of the same order of importance as the resistance of the pulmonary membrane to the diffusion of gas across it. Arguments are advanced to show that red cell resistance is of at least equal importance in the case of O₂ uptake. Submitted on February 15, 1957