The colour of blood varies with oxygen saturation due to the optical properties of the haem moiety. As the Hb molecule gives up O2 it becomes less permeable to red light and takes on a blue tint. Saturation is determined spectrophotometrically by measuring the 'blueness', utilising the ability of compounds to absorb light at a specific wavelength. The use of two wavelengths (650 and 940nm) permits the relative quantities of reduced and oxyhaemoglobin to be calculated, thereby determining saturation. The arterial pulse is used to provide time points to allow subtraction of the constant absorption of light by tissue and venous blood. The accuracy of pulse oximetry is within 2% above 70% SaO2.

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