The diffusing capacity and cardiac output were measured in 18 normal subjects and in 5 patients with pulmonary silicosis. These values were found to be in a close correlation with each other, suggesting that the overall reaction rate of CO with the red cell including diffusion and chemical reaction is almost identical among normal subjects. Thus, assuming that the overall reaction rate is constant, a simple method of estimating the contact time was tested. The overall rate was determined by

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... rring to both the data on reaction rate obtained by FUKUI and KAKIUCHI and the data on DLCO obtained by KRUHOFFER and FORSTER et al. The contact time obtained was 1.9sec, on an average, in normal subjects and the mean capillary blood volume was 134ml. In patients with silicosis the diffusing capacity was markedly reduced in parallel with the cardiac output, so that the calculated contact times were distributed within a range similar to those of normal subjects. The capillary blood volume showed a significant decrease, suggesting that the diffusion impairment may have been caused by the reduction of the capillary blood volume. As a result of the studies by ROUGHTON, FORSTER and their associates (ROUGH-TON, 1945a, b; FORSTER et al., 1957) , it has become clear that the CO uptake in the lung is limited, more or less, by the combination rate of CO with hemoglobin (Hb) in the red cell. Moreover, using the relation between the above reaction rate and PO2, they developed a method by which the CO diffusion rate across the alveolar membrane can be dealt with separately from its reaction rate with the red cell. In their method they assumed that the diffusion rate was related to diffusibility and surface area of the alveolar membrane. However, as long as the uptake rate of CO