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1 Department of Bioengineering, University of California California, San Diego, San Diego, CA, USA
* To whom correspondence should be addressed. E-mail: pjohnson{at}bioeng.ucsd.edu.
We tested the hypothesis that blood flow is distributed among capillary networks in resting skeletal muscle in such a manner as to maintain uniform end-capillary PO2. Oxygen tension in venules draining 2-5 capillaries was obtained by use of phosphorescence decay methodology in rat spinotrapezius muscle. For 64 post capillary venules among 18 networks in 10 animals the mean PO2 was 30.1 mmHg (range: 9.7 to 43.5 mmHg) with a coefficient of variation (standard deviation / mean) (CV) of 0.26. Oxygen levels of post-capillary venules within a single network or single animal, however, displayed a much smaller CV (0.064 and 0.094 respectively). By comparison, the CV of blood flow in 57 post-capillary venules of 17 networks in 9 animals was 1.27 with a mean flow of 0.011 ± 0.014 nl/s and range of 3.7 x 10-4 to 6.5 x 10-2 nl/s. Blood flow of post-capillary venules within single networks displayed a lower CV (mean: 0.51) while that in individual animals was 0.78. The results indicate that among venular networks, heterogeneity of oxygen tension is less than that of blood flow and within venular networks the heterogeneity of oxygen tension is much less than that of blood flow. In addition, we found that post-capillary PO2 was independent of flow among venules where both were measured. The results of this study may be attributable to three factors; O2 diffusion between adjacent capillaries and venules, structural remodeling in regions of lower PO2 and O2-dependent local control mechanisms. Our data also suggest that estimates of heterogeneity in local oxygen tension and volume flow based on pooled data from a number of animals overstate the variation within individual animals or local regions.
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