Fetal haemoglobin has a higher affinity for oxygen than adult haemoglobin. Therefore, at the same oxygen concentration or partial pressure, fetal haemoglobin has a higher percentage saturation, allowing oxygen to transfer from maternal blood to fetal blood across the placenta.
Feature | Fetal haemoglobin | Adult haemoglobin |
|---|---|---|
Oxygen affinity | Higher | Lower |
Oxygen dissociation curve | Shifted to the left | Shifted to the right relative to fetal haemoglobin |
Saturation at the same oxygen concentration | Higher | Lower |
Functional significance | Loads oxygen from maternal blood | Releases oxygen that can then enter fetal blood |
Both forms of haemoglobin show cooperative binding. When one oxygen molecule binds to a haem group, haemoglobin changes conformation, increasing the affinity of its remaining haem groups for oxygen. This produces the characteristic S-shaped, or sigmoidal, oxygen dissociation curve.
At the placenta, adult haemoglobin in maternal blood releases oxygen. Oxygen then diffuses across the placental exchange surface down its concentration gradient. Because fetal haemoglobin has the higher affinity, it binds this oxygen effectively even where oxygen concentration is relatively low.
Carbon dioxide also influences oxygen transport through allosteric binding. Increased carbon dioxide concentration reduces haemoglobin’s affinity for oxygen, causing greater oxygen dissociation, the Bohr shift. This helps maternal blood unload oxygen in placental tissues and also helps fetal blood unload oxygen in actively respiring fetal tissues.
A common misconception is that fetal haemoglobin “contains more oxygen” in every situation. The correct statement is that it has a higher percentage saturation at the same oxygen concentration; the amount carried still depends on the surrounding conditions.
Exam technique: For an AHL comparison, state the difference in affinity, identify the left shift of the fetal oxygen dissociation curve, and link this directly to oxygen transfer across the placenta. If asked to explain the curve’s S-shape, refer explicitly to cooperative binding rather than simply saying that haemoglobin “likes oxygen.”
