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Placental gas exchange: Bohr effect

Fetal supply of blood is facilitated by three factors:

  1. Oxygen concentration and partial pressure gradient within the feto-maternal circulatory system
  2. Fetal hemoglobin (HbF), which has higher affinity (lower P50 of 19mmHg) for oxygen then adult hemoglobin (HbA) with a P50 of 26.8mmHg. This facilitates transfer oxygen transfer from mother to fetus.
  3. Bohr effect

The Bohr effect is a physiological phenomenon referring to primarily the adult hemoglobin (HgA) affinity for OXYGEN (depending on the temperature, pH, pCO2, and 2,3-DPG concentration). This is in contrast to the Haldane effect, which describes primarily the HbA’s affinity for CARBON DIOXIDE (depending on oxygenated states such as lungs versus deoxygenated states such as tissue). In the feto-maternal circulatory system where the pH is lower and pCO2 is higher, there is a shift in the oxygen dissociation curve towards the right (e.g., decrease in HgA’s affinity for oxygen due to the Bohr effect) which favors oxygen dissociation from HgA and allows for delivery oxygen to HgF.


  1. Carter AM. Factors affecting gas transfer across the placenta and the oxygen supply to the fetus. J Dev Physiol. 1989;12(6):305. PubMed Link