Haemoglobin and oxygen delivery
Distinguish movement along a dissociation curve from a change in oxygen affinity.
What you will learn
- Explain loading and unloading at different oxygen partial pressures.
- Explain the effect of lower pH on affinity.
Partial pressure and saturation
Haemoglobin binds oxygen reversibly. Higher oxygen partial pressure supports loading, while lower partial pressure supports unloading. Actively respiring tissues use oxygen, maintaining conditions that favour delivery from the blood.
The oxygen dissociation curve relates saturation to partial pressure under specified conditions. A lower partial pressure moves the operating point along that curve; it should not automatically be described as a shift of the whole curve.
Affinity can change too
Increased carbon dioxide and lower pH can reduce haemoglobin's oxygen affinity, promoting unloading at a given oxygen partial pressure. This is associated with the Bohr effect. It helps match oxygen delivery to metabolically active tissues.
In an explanation, distinguish the oxygen partial-pressure gradient from a pH-related change in affinity rather than treating them as the same cause.
Worked example
At the same oxygen partial pressure, haemoglobin is less saturated in a more acidic sample. Explain what this indicates.
Show the worked solution
- Partial pressure is controlled, so the comparison is not simply movement along one unchanged curve.
- Lower saturation at that pressure indicates reduced oxygen affinity.
- The lower pH promotes oxygen unloading, consistent with the Bohr effect.
Answer Lower pH has reduced haemoglobin's oxygen affinity.
Common mistakes
- Lower partial pressure alone does not establish a pH-driven shift of the curve.
- Haemoglobin releases oxygen reversibly; it is not used up in delivery.
What can you explain now?
What happens to oxygen loading as blood encounters much lower oxygen partial pressure in tissue?
Compare with the explanation
Unloading is favoured
The reversible binding balance supports oxygen dissociation at lower oxygen partial pressure.
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