Muscle blood flow and distribution determine maximal VO₂ of contracting muscle

During repetitive contractions, the VO₂ of the dog gastrocnemius-plantaris muscle rose with the contraction frequency up to a maximal value and then decreased as contraction frequency was increased further. PVO₂ was constant over most of the contraction frequency range. Reducing perfusion pressure/blood flow reduced VO_2max with a constant PVO₂. During these maneuvers the diffusion conductance, DCO₂ (VO₂/PVO₂), changed with VO₂. Raising the perfusion pressure/flow with a pump increased VO₂ with a small rise in PVO₂ so that DCO₂ also increased. Removing tension from the muscle between contractions elevated VO₂ and DCO₂ without a change in perfusion pressure. Hypoxemia decreased VO₂ with a decrease in PVO₂; DCO₂ remained constant. A three-compartment mathematical model, based on microsphere measurements of regional flow, was used to illustrate how regional flow variations may exist, and how they are poorly revealed in the mixed whole-muscle venous blood. The model shows VO₂·g⁻¹ strongly related to flow. As VO₂·g⁻¹ increased as Q·g⁻¹ increased, extraction decreased, and DCO₂ increased.