What is the difference between hemoglobin's affinity for oxygen when comparing adult hemoglobin and fetal hemoglobin?

• A. Fetal Hb has a higher affinity for oxygen than adult Hb
• B. Fetal Hb and adult Hb have identical oxygen affinities
• C. Fetal Hb binds carbon monoxide more strongly than oxygen
• D. Fetal Hb has a higher affinity for oxygen than adult Hb

Answer: (D)

Fetal hemoglobin binds oxygen more tightly than adult hemoglobin, so it has a higher oxygen affinity. This difference mainly comes from structural changes: fetal Hb contains gamma subunits instead of beta subunits, which reduces how strongly 2,3-bisphosphoglycerate (2,3-BPG) binds to the molecule. Since 2,3-BPG lowers oxygen affinity, its reduced interaction with HbF shifts the oxygen-hemoglobin dissociation curve to the left. In the placenta, maternal blood delivers oxygen at a higher partial pressure, and HbF’s higher affinity allows efficient loading of O2 into fetal blood for delivery to fetal tissues. In contrast, adult HbA, with greater 2,3-BPG binding, has a lower oxygen affinity and releases oxygen more readily to tissues.