Fetal hemoglobin

Fetal hemoglobin, or foetal haemoglobin, (also hemoglobin F, HbF, or α₂γ₂) is the main oxygen transport protein in the human fetus during the last seven months of development in the uterus and persists in the newborn until roughly 6 months old. Functionally, fetal hemoglobin differs most from adult hemoglobin in that it is able to bind oxygen with greater affinity than the adult form, giving the developing fetus better access to oxygen from the mother's bloodstream.

In newborns, fetal hemoglobin is nearly completely replaced by adult hemoglobin by approximately 6 months postnatally, except in a few thalassemia cases in which there may be a delay in cessation of HbF production until 3–5 years of age. In adults, fetal hemoglobin production can be reactivated pharmacologically, which is useful in the treatment of diseases such as sickle-cell disease.

Oxygenated blood is delivered to the fetus via the umbilical vein from the placenta, which is anchored to the wall of the mother's uterus. The chorion acts as a barrier between the maternal and fetal circulation so that there is no admixture of maternal and fetal blood. Blood in the maternal circulation is delivered via open ended arterioles to the intervillous space of the chorionic plate, where it bathes the chorionic villi that carry umbilical capillary beds, thereby allowing gas exchange to occur between the maternal and fetal circulation. Deoxygenated maternal blood drains into open ended intervillous venules to return to maternal circulation. Due to the admixture of oxygenated and deoxygenated blood, maternal blood in the intervillous space is lower in oxygen than arterial blood. As such, fetal hemoglobin must be able to bind oxygen with greater affinity than adult hemoglobin in order to compensate for the relatively lower oxygen tension of the maternal blood supplying the chorion.

Fetal hemoglobin's affinity for oxygen is substantially greater than that of adult hemoglobin. Notably, the P50 value for fetal hemoglobin is lower than adult hemoglobin (i.e., the partial pressure of oxygen at which the protein is 50% saturated; lower values indicate greater affinity). The P50 of fetal hemoglobin is roughly 19 mmHg, whereas adult hemoglobin is approximately 26.8 mmHg. As a result, the "oxygen saturation curve", which plots percent saturation vs. pO₂, is left-shifted for fetal hemoglobin as compared to adult hemoglobin.

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