Oxygen Isotopic Exchange in Animal Respiration

Abstract

OXYGEN enriched with respect to ¹⁸O has been used to solve the problem of how far oxygen, expired by an animal in the form of carbon dioxide, is identical with that inspired as molecular oxygen. Using rats, and an artificial atmosphere containing oxygen equivalent in isotopic composition to water of 300 p.p.m. excess density, it was found that the expired carbon dioxide, which was dried and collected by refrigeration methods, contained oxygen isotopes in proportions corresponding to water with about 40 p.p.m. excess density, the error of measurement being 2 p.p.m. As the quantity of oxygen absorbed by the animal during each preliminary sweep-out was considerably greater than the total oxygen and carbon dioxide stored as oxy-hæmoglobin or bicarbonate, it can be accepted that the experimental results refer to the steady state.