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<i>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</i>
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1957
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Acute Lung InjuryEngineeringLung InflammationRelative ImportanceRed CellsGas Exchange ProcessPulmonary MembraneTransport PhenomenaLung HealthBiophysicsHuman Red CellsPulmonary CirculationLung DepositionPulmonary MedicineRespiration (Physiology)Chemical ReactionDiffusion ResistancePhysiologyPulmonary PhysiologyLung MechanicsTissue OxygenationRed Cell ResistanceMedicineChemical Kinetics
The study argues that red‑cell resistance to CO uptake is at least as important as pulmonary membrane resistance for oxygen absorption. An equation i/Dm + i/θVc = i/Dl was derived and solved graphically by measuring Dl at varying alveolar O2 tensions, using known θ values and Dl data from steady‑state and breath‑holding CO techniques to estimate Dm and Vc. The true diffusing capacity of the membrane (Dm) was found to be roughly twice the measured Dl, and the capillary blood volume (Vc) about 75 ml, indicating that red‑cell uptake resistance is comparable to membrane diffusion resistance. Submitted February 15, 1957.
An equation, i/Dm + i/θVc = 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 (Vc). By making measurements of Dl at different alveolar O2 tensions, thereby causing to vary, this equation can be solved graphically for Dm and Vc, which are assumed to be independent of O2 tension. Calculations of Dm and Vc 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. Vc 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