CONVECTION AS ONE OF THE LIMITING FACTORS OF HUMAN RESPIRATION DURINGNORMOXIC EXERCISE

Each of the pathways within respiration has been suspected of Limiting maximal performance, suggesting that O-2 transport may be affected by each single pathway. The use of the stable, isotopic O-2 molecules O- 16(2) and (OO)-O-16-O-18 is presented as a novel method for assessing respiration. Because of their different molecular weights, O-16(2) dif fuses 3% more rapidly than (OO)-O-16-O-18, whereas O-16(2) is convecti vely transported as rapidly as (OO)-O-16-O-18. This can be quantified by using the overall fractionation factor alpha(o). The more diffusion becomes limiting, the more O-16(2) is transported in preference to (O O)-O-16-O-18 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 O-2 transport. Six untrained subjects underwent nor moxic exercise on a cycle ergometer. Isotopic analysis was performed a t rest and during exercise loads of 50, 100, 150, 200, and 250 W using respiratory mass spectrometry. With increasing workload, a decrease i n alpha(o) from 1.0072 at rest to 1.0033 at 250 W was determined in al l subjects. On the basis of a serial resistance model of respiration, we concluded that, in our subjects, O-2 transport was increasingly aff ected by convection but decreasingly limited by diffusion. The relativ e contribution of convection to the entire resistance to O-2 flow rang ed from greater than or equal to 44.6% at rest to greater than or equa l to 74.6% at the most strenuous level of exercise, whereas the diffus ive pathways decreasingly contributed to resistance to O-2 flow by les s than or equal to 24% at rest and less than or equal to 11% at 250 W.