At the placental villus, carbon dioxide diffuses from fetal blood into maternal blood. What simultaneous change in the oxygen dissociation curves facilitates oxygen transfer from mother to fetus?

• A Maternal curve shifts left and fetal curve shifts right
• B Maternal curve shifts right and fetal curve shifts left ✓
• C Both maternal and fetal curves shift left
• D Neither curve shifts because pH changes cancel out

Explanation

This is the double Bohr effect. CO2 entering maternal intervillous blood lowers its pH, shifting the maternal curve rightward and promoting O2 unloading. Simultaneously, loss of CO2 from fetal blood raises fetal pH, shifting the fetal curve leftward and increasing its O2 uptake. Option A describes the reverse and would impede transfer, while a single-sided shift would be insufficient, which is why the bidirectional pH change is the accepted explanation.

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

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