Haldane and indifferent gases: O2 secretion or CO excretion?

The method that Haldane used in 1911, when he claimed that O2 comes to be secreted into the pulmonary capillary blood after a few days at altitude, required that CO be an indifferent gas except for one property: it combines with Hb. But that is now known not to be true. CO is formed when Hb is katabolised, it reacts with many substances in the body, and it is involved in transmissions between cells. If, instead of supposing that CO is an indifferent gas, one proceeds from Barcroft's observation that PaO2 remains equal to PAO2 on going to altitude, the conclusion from Haldane's observations must be that he showed that CO has come to be actively excreted from the body after a few days at altitude. And that presumably happened because, as Killick (J. Physiol., London, 107: 27-44, 1948) suggested, Haldane's method required that the subject be repeatedly exposed to CO. The transport of CO around the body by Hb, and the possible effects on this, and on DLCO, of active excretion of CO, have to be considered.