Haemoglobin adaptations depend on reversible gas binding, cooperative oxygen loading and affinity differences between adult haemoglobin, foetal haemoglobin and altitude-adapted forms.
Coverage 2018–2025 · Updated 15 Jul 2026
Adult and foetal haemoglobin are four-subunit proteins whose haem groups bind oxygen reversibly and cooperatively, but foetal haemoglobin has a higher oxygen affinity.
Binding of one oxygen molecule changes haemoglobin conformation and increases the affinity of remaining haem groups, supporting rapid loading at high oxygen partial pressure. Falling partial pressure reverses the process and supports unloading.
Carbon dioxide binds allosterically at sites separate from haem and can alter oxygen affinity. Foetal haemoglobin's higher affinity gives its dissociation curve a leftward position relative to adult haemoglobin, enabling oxygen transfer from maternal blood at the placenta.
At the same placental oxygen partial pressure, foetal haemoglobin can have a higher percentage saturation than maternal adult haemoglobin, favouring net oxygen movement into foetal blood.
Higher affinity aids foetal loading but affinity must still fall enough for tissue unloading. Carbon dioxide does not compete with oxygen for the iron atom of the haem group; its effect is allosteric.
This objective is assessed through structured response, commonly using Outline / Describe / Suggest.
Outline / Describe / Suggest
Build the answer around this relationship: Oxygen binds reversibly to haem groups containing iron in haemoglobin.
Describing foetal haemoglobin as having lower affinity instead of a left-shifted, higher-affinity curve.
Representative question
Suggest how changes in hemoglobin could help humans become better adapted to living at high altitude.
Haemoglobin increases oxygen transport because oxygen is poorly soluble in plasma. Reversible and cooperative binding allow loading at high pO2 and unloading at low pO2. High carbon dioxide lowers pH and causes the Bohr shift, reducing affinity and promoting oxygen release in active tissues. Dissociation curves show these changes through sigmoid shape and left/right shifts.
