The closed rebreathing circuit has been widely used for residual air measurement (1, 2, 3, 4, 5, and others). The essential features of the apparatus and procedure were the same in all. A spirometer filled with high oxygen (or with a hydrogen-containing) gas mixture was attached to the test subject. The carbon dioxide was removed with soda lime or other alkali in the circuit and the subject breathed for a period of time until the nitrogen (or hydrogen) was redistributed in the circuit. The residual lung gas volume was calculated from the amount of redistributed gas.In most of the work it was assumed that at the end of five or more minutes of quiet breathing the gas in lungs and spirometer was uniform in composition, except for a slight excess concentration of inert gases in the lungs. This excess was assumed to be the same as that occurring during the breathing of room air. On the basis of these assumptions, the net change in spirometer gas concentration was considered the same as that in the lungs during the rebreathing. By actual measurement of the gas concentration in the various parts of the spirometer system and the alveolar gas, Lassen, Cournand, and Richards (6) concluded in the study of normal subjects that these assumptions were not valid. Rather than an excess, they found a lower concentration of nitrogen in the lungs than in the spirometer at the end of a breathing period. They explained these findings as due to the constantly increasing nitrogen concentration which resulted from the steadily diminishing spirometer volume. This effect they called the "oxygen storage" effect. To correct for it, they introduced alveolar air concentrations into the calculation and found that this correction made a
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