EXERCISE O-2 TRANSPORT MODEL ASSUMING ZERO CYTOCHROME PO2 AT VO2MAX

An analogy is drawn between cytochrome aa(3) function and a polarograp hic cathode at which the potential of -0.6 V captures all O-2 diffusin g to the surface, achieving maximal O-2 consumption (Vo(2max)) by elim inating O-2 backpressure and outward diffusion from the surface, defin ed herein as zero surface Po-2. The relationship of O-2 consumption (a s %Vo(2max)) to muscle venous, myoglobin, and cytochrome Po-2 is model ed assuming that cytochrome aa(3) Po-2 reaches zero at Vo(2max), incor porating published data on the profile of leg venous Po-2, pH, and blo od lactate vs. work. Equations describe hemoglobin and myoglobin O-2 d issociation and the Bohr effect of acid on O-2 unloading. The O-2 grad ient from capillary blood to cytochrome aa(3) is assumed to be proport ional to O-2 consumption. The model suggests that 1) to extract 75% of the O-2 from myoglobin at Vo(2max), myoglobin must lie 90% down the O -2 gradient from capillary to cytochrome; 2) the Bohr effect adds 15-3 0% to Vo(2max) and keeps venous Po-2 almost constant as work rises fro m 60 to 100% of Vo(2max); and 3) in steady heavy work, the rising arte rial lactate may impede lactate excretion from muscle, reduce anaerobi c ATP generation, and shift the energy balance toward aerobic metaboli sm. The zero Po-2 hypothesis facilitates modeling and may be the key t o understanding the physiological limitation of work.