Oxygen transport
Almost all oxygen travels bound to haemoglobin in red cells; how tightly it binds changes with the tissue's needs.
Each haemoglobin molecule carries four oxygen molecules, and a litre of arterial blood holds about 200 ml of oxygen, over 98% of it bound. The binding curve is S-shaped: haemoglobin loads fully in the lungs yet releases oxygen readily where partial pressure is low. Acid, heat and carbon dioxide, all produced by working tissue, shift the curve to release more (the Bohr effect).
Pulse oximeters estimate the percentage of haemoglobin carrying oxygen; 95 to 100% is normal at sea level. Anaemia lowers the amount of haemoglobin, so total oxygen delivery falls even when saturation reads normal, which is why a saturation reading alone can mislead.
Carbon monoxide binds haemoglobin about 240 times more tightly than oxygen, and standard oximeters cannot tell the difference.
Key facts
| Fact | Value |
|---|---|
| Normal oxygen saturation | 95–100% |
| Oxygen per litre of arterial blood | ≈200 ml |
| Haemoglobin (adult male) | ≈130–170 g/L |