AJP - Regulatory, Integrative and Comparative Physiology, Vol 272, Issue 6 1874-R1879, Copyright © 1997 by American Physiological Society
Convection as one of the limiting factors of human respiration during normoxic exercise
H. Heller and K. D. Schuster
Department of Physiology, University of Bonn, Germany.
Each of the pathways within respiration has been suspected of limiting
maximal performance, suggesting that O2 transport may be affected by each
single pathway. The use of the stable, isotopic O2 molecules 16O2 and
16O18O is presented as a novel method for assessing respiration. Because of
their different molecular weights, 16O2 diffuses 3% more rapidly than
16O18O, whereas 16O2 is convectively transported as rapidly as 16O18O. This
can be quantified by using the overall fractionation factor alpha O. The
more diffusion becomes limiting, the more 16O2 is transported in preference
to 16O18O and alpha O is increased to 1.03. By contrast, the more
respiration is limited by convection, the closer alpha O comes to 1.0
during the entire O2 transport. Six untrained subjects underwent normoxic
exercise on a cycle ergometer. Isotopic analysis was performed at rest and
during exercise loads of 50, 100, 150, 200, and 250 W using respiratory
mass spectrometry. With increasing workload, a decrease in alpha O from
1.0072 at rest to 1.0033 at 250 W was determined in all subjects. On the
basis of a serial resistance model of respiration, we concluded that, in
our subjects, O2 transport was increasingly affected by convection but
decreasingly limited by diffusion. The relative contribution of convection
to the entire resistance to O2 flow ranged from > or = 44.6% at rest to
> or = 74.6% at the most strenuous level of exercise, whereas the
diffusive pathways decreasingly contributed to resistance to O2 flow by
< or = 24% at rest and < or = 11% at 250 W.
