Nonlinear regulation of capillary perfusion in relation to ambient pO₂ changes in skeletal muscle

To study the process of O₂ transport to tissue, we investigated how capillary perfusion is controlled in response to changes in tissue O₂ levels in skeletal muscle. Capillary red blood cell (RBC) velocity and perfused capillary recruitment were measured in rabbit tenuissimus muscle at various ambient oxygen tensions (pO₂) by intravital microscopy. Both RBC velocity and capillary recruitment significantly decreased as the pO₂ level of the suffusate was increased, and the relationship between capillary perfusion, calculated from the velocity and recruitment data, and the pO₂ level of the suffusate clearly yielded a nonlinear correlation that fitted a sigmoidal curve. Capillary perfusion dramatically decreases or increases above or below a suffusate pO₂ level of around 40 Torr, where the O₂ dissociation curve of hemoglobin changes slope. These findings support the hypothesis that microvasculature possesses an intrinsic, effective flow-control mechanism by sensing the metabolic demands of tissue, intimately related to the O₂ saturation of hemoglobin.