= Oxygen consumption
{title2=$\dot V_{\mathrm O_2}$}
\b[<Oxygen consumption>] is the rate at which a system uses molecular <oxygen> in its chemical processes. In aerobic <cellular respiration>, oxygen participates in reactions supporting energy metabolism. The system boundary might be an isolated preparation, one tissue or the whole body; its definition determines which inflows, outflows and stored oxygen belong in the balance.
For a tissue supplied by <blood flow> $Q$, with compatible <arterial oxygen content> $C_a$ and <venous oxygen content> $C_v$, conservation gives
$$
\frac{dM_{\mathrm O_2}}{dt}=Q C_a-Q C_v-\dot V_{\mathrm O_2},
$$
when all terms use the same oxygen-amount or reference gas-volume basis. At steady oxygen storage,
$$
\boxed{\dot V_{\mathrm O_2}=Q(C_a-C_v)}.
$$
This is the oxygen form of the <Fick principle>. It measures net oxygen removal across the stated territory, not oxygen delivery alone.
An original example takes $Q=4.00$ L/min, $C_a=200$ mL oxygen/L and $C_v=140$ mL oxygen/L. Consumption is 240 mL oxygen/min, while arterial delivery is 800 mL/min. If tissue oxygen storage increases by an equivalent 10 mL/min, the same measured inflow–outflow difference instead permits consumption of 230 mL/min. Neglecting accumulation is an assumption to test, especially during transitions.
<Respirometry> supplies another route: measured gas inflow and outflow, or oxygen decline in a suitable closed chamber, constrain oxygen use. Chamber volume, leakage, sensor response and changes in stored gas or dissolved oxygen affect that inference. A falling oxygen signal is meaningful only after its amount basis and the system boundary are established.
A primary comparison of cardiac-output methods explicitly distinguished thermodilution from Fick output calculated using estimated oxygen uptake.<Thermodilution vs Estimated Fick Cardiac Output Measurement in Clinical Practice by Alexander R. Opotowsky and others>[Abstract, Importance and Exposures]{ref} An assumed consumption value is not the same observation as direct measurement. Whole-body calculations also require <mixed venous oxygen content>, while a regional venous sample belongs with that region's flow. Report the measurement or estimation method alongside the resulting rate.
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