Red blood cell transit time in man: theoretical effects of capillary density

These data indicate that through the reduction in exercise time and recruitment of trained subjects, the exercising muscle in the human dynamic knee-extension model can reach even higher work rates, VO2, and Q than previously reported (Andersen & Saltin, 1985; Andersen et al., 1985; Rowell et al., 1986). Despite these high muscle Q, the achievement of high O2 extractions is possible (Richardson et al., 1993). Previously, this was attributed almost exclusively to the elevated WRMAX, and it was therefore concluded that O2 extraction is not limited by the high Q to any greater extent than in conventional two legged cycle ergometry (Richardson et al., 1993). It is now apparent from the analysis of the data in this paper that it is possible that a difference in capillary density between the subjects in the original studies and the present research may have played a role in the increase in O2 extraction with increasing muscle Q. Although, it should be recognized that a) the capillary density necessary to reduce red cell transit time suitably to match the range of measured femoral venous hemoglobin saturation is high (Gayeski et al., 1988), however it is within the measured values for man (Brodal et al., 1977), and b) where comparable, during low WR knee-extensor exercise and whole body VO2MAX, subjects across the studies did not differ. This would be expected if capillary density differed greatly (Brodal et al., 1977). It can therefore be concluded that both the rapid protocol during knee-extensor exercise and the potential increased capillarity of the trained subjects in the present study may have combined to produce the amplified physiologic and WR responses.