Fetal hemoglobin (HbF, α2γ2) has a left-shifted oxygen dissociation curve compared to adult HbA. At the placenta, the Bohr effect facilitates O2 transfer from mother to fetus via which mechanism?

• A Maternal hyperventilation raises maternal PaO2, directly increasing O2 delivery to fetal blood
• B Fetal CO2 transfer to maternal blood acidifies maternal erythrocytes (Bohr effect), shifting maternal ODC right and releasing O2 to fetal blood which simultaneously becomes more alkaline and shifts its ODC left — double Bohr effect ✓
• C HbF has lower affinity for 2,3-DPG than HbA, directly increasing its O2 affinity by displacing DPG from beta chains
• D Placental secretion of carbonic anhydrase into fetal blood accelerates CO2 hydration, lowering fetal pH and driving O2 off HbF

Explanation

The double Bohr effect optimally transfers O2 at the placenta: CO2 produced by the fetus diffuses into maternal blood, acidifying maternal erythrocytes → Bohr effect shifts maternal HbA O2 affinity curve rightward, releasing O2. Simultaneously, maternal blood becoming more alkaline (losing CO2) shifts fetal HbF curve leftward, increasing O2 uptake. This synergistic bidirectional Bohr effect maximizes transplacental O2 transfer. Option C is true (HbF gamma chains have lower 2,3-DPG binding than beta chains, so less DPG effect and higher O2 affinity) but does not describe the Bohr-mediated mechanism asked. Option D describes the reverse of what actually happens.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

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Written and medically reviewed by the StethoPrep medical team.