Fetal Haemoglobin

A fetus cannot breathe air; it obtains oxygen from its mother's blood at the placenta. The maternal and fetal blood do not mix; oxygen diffuses from the mother's blood into the fetal blood across the placenta.By the time maternal blood reaches the placenta, it has already given up some of its oxygen, so the pO₂ at the placenta is fairly low. If the fetus had the same haemoglobin as its mother, its haemoglobin could not become well saturated there.The fetus therefore has fetal haemoglobin, which has a higher affinity for oxygen than adult haemoglobin. Its oxygen dissociation curve is to the left of the adult curve: at any pO₂, fetal haemoglobin is more saturated.At the placenta, where pO₂ is low, the mother's haemoglobin releases oxygen (helped by the Bohr effect, as the fetal blood delivers CO₂ to the maternal blood). Fetal haemoglobin, with its higher affinity, picks this oxygen up. Oxygen is transferred from maternal to fetal haemoglobin.Fetal haemoglobin has a different structure: it has two α chains and two γ (gamma) chains instead of two β chains. After birth, the baby breathes air, and fetal haemoglobin is gradually replaced by adult haemoglobin during the first months of life. This is important because adult haemoglobin, with its lower affinity, releases oxygen more readily in the tissues.Worked example: at a placental pO₂ of 4 kPa, adult haemoglobin might be 60% saturated and fetal haemoglobin 80% saturated, so fetal haemoglobin holds 20% more of its maximum oxygen there.