Investigation of the human oxygen transport system during conditions of rest and increased oxygen consumption by means of fractionation. Effects of oxygen isotopes.

The aim of the study was to assess various pathways of the oxygen transport system at rest and ergometer work by the utility of information derivable from different behaviour of the isotopic oxygen molecules 16O2 and 16O18O during transport. 6 healthy humans were studied at rest and at different levels of ergometer work, ranging from 50 to 250 W. Isotope analysis was performed by applying a reference technique. Samples of inspiratory and expiratory gas, taken during steady state conditions, were analysed by a respiratory mass spectrometer on their 16O18O/16O2 ratios by comparing them with a reference gas of appropriate composition. Ventilatory minute volume, oxygen consumption and end-expiratory oxygen partial pressure were also measured. At rest, the delta IU-value quantifying the isotope effect of the overall oxygen transport amounted to 0.74 +/- 0.08%. During ergometer work, the fractionation factor steadily decreased with increasing oxygen consumption, yielding the regression line delta IU [%] = 0.74 -1.3.10(-4).VO2, where VO2 is oxygen consumption given in ml/min. With respect to oxygen transport from inspiratory gas to tissue, this result suggests: convective processes of oxygen transport become more limiting with increasing oxygen consumption, whereas diffusion does not become a major limiting step up to oxygen consumption rates of 3600 ml/min, otherwise a reverse relationship between fractionation factor and oxygen consumption should have been found.