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An experiment involving simultaneous pulmonary uptake of nitrous oxide and cyclopropane is analyzed for compatibility with the perfusion-limited multicompartment model of inert gas uptake. These test gases have approximately equal solubilities in body watery tissues (blood, muscle, viscera, etc.) but an approximately 1-to-8 ratio of solubility in fat. Therefore, the measured difference in time course of pulmonary uptake provides a sensitive test of the mechanism of adipose tissue uptake of these gases. The experimental data could not be accounted for by the model, assuming plausible compartment parameters, or by using derived compartment parameters. Expansions of the model to account for the discrepancy are suggested. gas uptake in body; fat-soluble gas uptake; multiple gas uptake; fat perfusion; adipose tissue perfusion; blood flow to fat; perfusion of fat; inert gas kinetics; tissue gas exchanges Submitted on February 3, 1964

An approximately constant 5% difference in alveolar concentration of nitrous oxide and cyclopropane exists when these two gases are administered simultaneously to human subjects. This difference in uptake cannot be fully explained within the traditional framework of a perfusion-limited, multi-compartment model of inert gas exchange. It is proposed that this difference reflects direct diffusion from lean to neighboring adipose tissue through distances of the order of 1 mm. The diffusional rate of cyclopropane uptake into adipose tissue is initially large relative to perfusional uptake. The two rates eventually become and remain comparable as both decrease to zero. Implications of these results for deduction of blood flow to body adipose tissue by gas uptake measurement, and for utilization of capillary exchange surface by fat-soluble gases in adipose tissue are discussed. compartment model generalization; gas uptake in body; inert, fat-soluble gas uptake; kinetics of gas exchange in body; body uptake of inert gases; fat-soluble gas uptake; distribution kinetics of gases in body Submitted on February 3, 1964