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Figure 3: Systemic arterial chemoreceptors are ideally located anatomically to measure any mismatch but are in the wrong place to measure metabolic rate or metabolically produced CO2. Diagram of typical values of cardiac output, systemic arterial PaCO2, and mixed PvCO2, and of blood CO2 content at rest (a) and maximum exercise (b), with data from multiple sources. For clarity, the corresponding numbers for metabolic rates are omitted. Comparison of (a) and (b) shows that systemic arterial chemoreceptors are in the wrong place to measure metabolically produced CO2, because metabolically produced CO2 is carried via the systemic veins from muscle to air without PaCO2 rising.