Mixed venous oxygen content is the oxygen content of blood after the returning venous streams have mixed, representing whole-body venous return to the right side of the heart. It is the venous quantity required with arterial oxygen content in the whole-body Fick principle. A single regional vein need not have this content: different tissues remove different amounts of oxygen and return different blood flow rates.
The mixing rule follows from conservation of mass. If streams have blood flow rates and venous oxygen content values , with no oxygen exchange during mixing,For two constructed streams, mL/min at mL of oxygen per mL of blood and mL/min at mL/mL carry and mL/min of oxygen. Their mixture has content mL/mL, not the unweighted mean mL/mL. The correct weights are volume blood flow rates.
The right atrium receives several returns before blood passes through the right ventricle and pulmonary trunk.[ref] A downstream sample can represent their mixture only to the extent that the streams have actually mixed and the sampling interval represents the desired state. Replacing that sample with one taken from a particular upstream vein introduces a representativeness assumption. Likewise, averaging oxygen saturation values is not generally an oxygen-content balance when hemoglobin concentrations differ. The whole-body balance is about amounts of transported oxygen, not averages of instrument displays.
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