The fetal hemoglobin (HbF) oxygen dissociation curve is shifted to the LEFT compared to adult HbA. What is the primary physiological mechanism of this leftward shift, and why is it important?
- A HbF gamma-chains bind 2,3-DPG less avidly than HbA beta-chains, reducing Bohr effect and increasing O2 affinity for placental O2 transfer ✓
- B HbF has higher CO2 affinity, displacing O2 from binding sites
- C HbF has more heme groups per molecule than HbA, increasing O2 carrying capacity
- D Fetal blood pH is more alkaline due to higher CO2 production, causing intrinsic leftward shift
Correct answer: A. HbF gamma-chains bind 2,3-DPG less avidly than HbA beta-chains, reducing Bohr effect and increasing O2 affinity for placental O2 transfer
Explanation
HbF contains gamma (γ) subunits instead of beta (β) subunits. Gamma-chains bind 2,3-diphosphoglycerate (2,3-DPG) with much lower affinity than beta-chains, so HbF has relatively high O2 affinity compared to HbA. This leftward shift in the oxygen-hemoglobin dissociation curve allows HbF to load O2 at the low PO2 of the placenta (~30 mmHg on the fetal side), effectively capturing O2 released by maternal HbA (which has a rightward shift due to the double Bohr effect). This is essential for fetal oxygenation.
Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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