Mechanisms which control VO2 near VO2max: an overview.

The symposium, "Mechanisms Which Control VO2 Near VO2max," led to a general agreement that there is a variable impediment to the movement of O2 from the interior of the red cell to the interior of the mitochondrion. By changing the variables associated with O2 delivery or by altering the conditions for muscle contractions, the effective O2 diffusing capacity for muscle can be altered as well. Because it is often measured as the ratio of VO2/PvO2, it was suggested that this be referred to as O2 conductance rather than diffusing capacity. In contrast to the wide range of O2 conductance values found by the symposium contributors, a very narrow range of O2 extraction values was found when VO2 was graphed against O2 delivery. The only experimental values that departed from this relationship to any degree were those where hemoglobin function was altered or if blood flow was forced to extraordinary high levels by a pump. The limits for VO2 in contracting isolated muscle are set not only by O2 supply but by O2 demand associated with stimulus patterns. Other intriguing and perhaps useful questions are: 1) What is the relative contribution of such factors as diffusional shunting, flow heterogeneity, red cell transit time, etc., to the apparent O2 conductance? 2) How is blood flow to contracting muscle controlled? 3) How is contractile force adjusted to energy supply?