At 32 weeks gestation, fetal haemoglobin (HbF) has a P50 of 19 mmHg compared to adult HbF P50 of 27 mmHg. This left-shifted oxygen dissociation curve of HbF serves which physiological function in the placenta?

• A HbF's left shift reduces oxygen delivery to fetal tissues, redirecting O2 to placental trophoblasts for anaerobic glycolysis
• B HbF's higher affinity compensates for reduced cardiac output in the fetus by maintaining saturation at lower haemoglobin concentrations
• C HbF's P50 of 19 mmHg means it unloads O2 more efficiently at peripheral fetal tissues due to the steep portion of the curve at low PO2
• D HbF's higher O2 affinity allows it to extract oxygen from maternal HbA at the placental intervillous space even when maternal PaO2 is only 30–35 mmHg, ensuring adequate fetal oxygenation via the double Bohr effect ✓

Explanation

The placenta operates via a double Bohr effect: at the intervillous space, maternal blood releases CO2 to fetal blood and takes up fetal CO2, shifting maternal HbA curve right (reduced affinity) and fetal HbF curve left (increased affinity). This simultaneous shift facilitates O2 transfer from maternal to fetal blood even at the relatively low PO2 of placental blood (maternal intervillous PO2 ~30–35 mmHg). HbF's higher affinity (lower P50) allows it to load O2 where maternal HbA begins to unload—a thermodynamic advantage exploited by the double Bohr effect. HbF does NOT improve unloading at peripheral tissues; that is a disadvantage compensated by the fetal polycythaemia.

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

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