Interpreting oxygen saturation
Plotting the percentage saturation of haemoglobin against the partial pressure of oxygen (pO2) produces an oxyhaemoglobin dissociation curve.
Percentage saturation indicates the proportion of available oxygen-binding sites occupied by O2. At complete saturation, oxygen occupies all four haem groups in each haemoglobin molecule.
- When pO2 is low, few oxygen-binding sites are occupied and oxygen is readily released.
- In the steep region, relatively small changes in pO2 produce much larger changes in saturation.
- When pO2 is high, oxygen loading is strongly favoured and haemoglobin approaches full saturation.
What changes in pO2 mean
The graph can be followed in either direction. Rising pO2 favours association of oxygen with haemoglobin, while falling pO2 favours dissociation and release.
| pO2 | Behaviour of haemoglobin | Biological consequence |
|---|---|---|
| Low | Saturation is low and bound oxygen is readily lost. | Oxygen can be supplied to respiring tissues where oxygen availability has fallen. |
| Intermediate | A modest change in pO2 produces a much larger change in saturation. | A considerable amount of oxygen can be loaded or released over a relatively small pO2 change. |
| High | Most binding sites are occupied and saturation is close to its maximum. | Blood passing through the lungs can load a large amount of oxygen. |
Exam Tip: Interpret the steep region in both directions. If pO2 rises, saturation increases sharply; if pO2 falls, a large proportion of the bound oxygen can be released.