Fetal hemoglobin (HbF) has a higher oxygen affinity than adult hemoglobin (HbA). What is the physiological basis of this difference and why is it critical for fetal oxygen acquisition?

• A HbF contains γ-chains instead of β-chains; γ-chains bind 2,3-BPG less avidly, resulting in less allosteric inhibition and higher O2 affinity — this left-shifts the curve to capture O2 from maternal HbA at the placenta ✓
• B HbF has a lower Bohr effect coefficient, so it retains oxygen better in acidic conditions of the placenta
• C HbF contains δ-chains that have intrinsically higher O2-binding capacity per heme group
• D HbF has higher O2 affinity because it lacks the T→R allosteric transition, existing in a constitutively relaxed (R) state

Explanation

HbF is composed of two α-chains and two γ-chains (α2γ2), while HbA has β-chains (α2β2). The γ-chain has reduced affinity for 2,3-bisphosphoglycerate (BPG) compared to β-chains; since 2,3-BPG stabilizes the deoxy (T) state of hemoglobin and reduces O2 affinity, less 2,3-BPG binding in HbF means it remains in the high-affinity (R) state more easily. This left-shifts HbF's oxygen dissociation curve compared to HbA, allowing HbF to extract oxygen from maternal HbA across the placenta where PO2 is relatively low (~40 mmHg). Option B is incorrect; the Bohr effect actually facilitates O2 transfer (double Bohr effect) but via a different mechanism. Options C and D are structurally incorrect.

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

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