Oxyhaemoglobin

A saturation of 100% is only achieved if haemoglobin is exposed to a PO₂ of 33.2 kPa. If it is exposed to a lower partial pressure, the percentage falls. This can be represented graphically to produce a curve .

The rapid uptake of oxygen is represented by the initial steep slope of the curve, where small changes in PaO₂ cause large changes in the saturation. Beyond a PO₂ of 6.6 to 8.0 kPa, the curve becomes flatter as little further saturation occurs despite increases in PO₂. In the normal situation a PaO₂ of approximately 13.3 kPa results in a saturation of around 97%.

This curve is called the oxyhaemoglobin dissociation curve, although it clearly relates also to the association of oxygen with haemoglobin.