Red Blood Cell Transit Time in Man: Theoretical Effects of Capillary Density

Abstract

The pioneering development of the knee-extensor exercise model opened a new dimension in the study of human muscle physiology allowing the measurement of blood flow (Q), oxygen uptake (VO₂), and work rate in an isolated muscle (Andersen et al., 1985). Original studies using this exercise paradigm reported reduced O₂ extractions presumably as a consequence of the greater maximal muscle Q (Andersen & Saltin, 1985). Although this human in situ model has elevated the maximum recorded muscle Q in man from the previously recorded values of 60-100 ml• 100g • min^-1 (Mellander, 1981) to values of 250-300 m1-100g • min^-1, it was still not apparent that this was maximal human muscle Q (Rowell, 1988). A recent experiment was performed to determine if human muscle Q could be elevated above these previously reported values during knee-extensor exercise by recruiting athletic subjects and utilizing a rapidly incremented protocol (Richardson et al., 1993). In this study quadriceps muscle Q(x ± SE, 385 ± 26 ml • min^-1100g), VO_2max (60.2 ± 5.8 ml • min^-l• 100g^-1), and work rate (4.3 ± 0.3 W • 100g^-1) were by far the highest yet recorded. Despite these being the highest recorded muscle blood flows in man, O₂ extraction at Vo_2max did not appear to be compromised to any greater extent than in conventional cycle exercise (Richardson et al., 1993). Extraction at any given Vo₂ is determined largely by convective O₂delivery, capillary surface area available for O₂ uptake, and the resultant time available for O₂ release (Honig et al., 1991). It is the objective of this paper to focus on the third of these factors which_. equates to red blood cell (RBC) transit time during these high human muscle Q (Richardson et al., 1993). Previously, it has been demonstrated theoretically that transit time and its variability are major determinants of O₂ extraction from red cells at exercise (Gayeski et al., 1988).