(A) is not correct but (R) is correct
Fetal hemoglobin’s higher O2 affinity enables efficient oxygen transfer from maternal blood to fetal blood despite low placental O2 saturation, making Reason (R) the correct explanation for Assertion (A).
Correct Answer
Both (A) and (R) are correct and (R) is the correct explanation of (A).
Assertion Analysis
Assertion (A) states that maternal blood O2 saturation is quite low in the placenta, yet O2 transfers readily to fetal blood. This is correct because maternal arterial blood enters the placenta fully saturated (~100%), but deoxygenates significantly in placental capillaries, reaching venous saturation of ~70-80% (pO2 ~30-40 mmHg) due to O2 diffusion to the fetus. Fetal blood achieves ~80% saturation at this low pO2, confirming efficient transfer.
Reason Analysis
Reason (R) states fetal hemoglobin (HbF) binds BPG (2,3-bisphosphoglycerate) less strongly, giving higher O2 affinity than adult hemoglobin (HbA). This is correct: HbF’s γ-chains reduce BPG binding by ~10-fold compared to HbA’s β-chains, shifting the O2 dissociation curve leftward (P50 ~19 mmHg for HbF vs. ~26 mmHg for HbA).
Option Evaluation
| Option |
Evaluation |
| Both (A) and (R) correct; (R) explains (A) |
Correct. Low placental maternal saturation would limit fetal uptake if HbF affinity matched HbA, but HbF’s BPG insensitivity ensures higher O2 loading at low pO2 . |
| Both correct but (R) not explanation |
Incorrect; HbF affinity directly enables the transfer described in (A) . |
| (A) correct, (R) incorrect |
Incorrect; (R) is factually true . |
| (A) incorrect, (R) correct |
Incorrect; (A) accurately describes placental physiology . |
Introduction to Placental Oxygen Transfer
Placental oxygen transfer relies on fetal hemoglobin’s higher O2 affinity despite low maternal blood O2 saturation in placenta. This mechanism ensures fetal oxygenation even at reduced partial pressures, critical for biology exams like NEET.
Maternal Blood O2 Saturation in Placenta
Maternal arterial blood reaches the placenta at high saturation but drops to 70-80% in venous blood (pO2 ~30 mmHg) as O2 diffuses across the barrier. This “quite low” saturation still transfers O2 readily to fetal blood due to affinity differences.
Fetal Hemoglobin and BPG Role
Fetal hemoglobin (HbF) binds BPG less strongly than adult HbA, maintaining high O2 affinity (left-shifted curve). At placental pO2, HbF saturates ~80% vs. HbA’s ~50%, driving passive transfer.
Diagram showing O2 dissociation curves: HbF vs. HbA at placental pO2.
Why This Matters for Exams
This assertion-reason pair tests understanding of respiratory physiology adaptations. Key phrase “fetal hemoglobin binds BPG less strongly” directly explains efficient placental O2 transfer.
