GLOSSARY

The oxygen–haemoglobin dissociation curve is a graph of haemoglobin saturation (SO₂) against oxygen partial pressure (PO₂), with an S-shaped (sigmoid) form.

Key Points

• PO₂ about 100 (arterial): saturation about 97–98 percent.
• PO₂ about 60: saturation still about 90 percent.
• PO₂ about 40 (venous at rest): saturation about 75 percent, meaning only about 25 percent of the oxygen has been released.
• P50: the PO₂ at 50 percent saturation, about 26–27 mmHg.

Importance of the S Shape

• Flat upper part (plateau): even with a considerable fall in PO₂ (for example at moderate altitude), saturation hardly falls, providing a safety margin for oxygen loading in the lung.
• Steep part: a small change in PO₂ in the tissues causes large release of oxygen.