What is the P50 of fetal hemoglobin at term?

What is the P50 of fetal hemoglobin at term?

• A. 15
• B. 20
• C. 27
• D. 30

Answer: (B)

P50 is the oxygen tension at which hemoglobin is 50% saturated with oxygen. A lower P50 means higher affinity for O2. Fetal hemoglobin has a higher oxygen affinity than adult hemoglobin because its gamma chains substitute for the beta chains and bind 2,3-BPG less, shifting the dissociation curve to the left. This makes it easier for HbF to take up oxygen from the maternal blood in the placenta, where the oxygen tension is relatively low. At term, HbF is still prominent, giving a P50 of about 20 mmHg, which is lower than the P50 of adult Hb (around 27 mmHg). So the value around 20 mmHg best reflects HbF’s higher oxygen affinity at term.

P50 is the oxygen tension at which hemoglobin is 50% saturated with oxygen. A lower P50 means higher affinity for O2. Fetal hemoglobin has a higher oxygen affinity than adult hemoglobin because its gamma chains substitute for the beta chains and bind 2,3-BPG less, shifting the dissociation curve to the left. This makes it easier for HbF to take up oxygen from the maternal blood in the placenta, where the oxygen tension is relatively low. At term, HbF is still prominent, giving a P50 of about 20 mmHg, which is lower than the P50 of adult Hb (around 27 mmHg). So the value around 20 mmHg best reflects HbF’s higher oxygen affinity at term.