Foetal Haemoglobin

• A foetus obtains its oxygen from its mother’s blood, via the placenta.
• The haemoglobin of a foetus has a higher affinity for oxygen than adult haemoglobin. This means that, at the same partial pressure of oxygen,  foetal blood is better at absorbing oxygen than its mother’s blood.
• By the time the mother’s blood reaches the placenta, its oxygen saturation has decreased because oxygen has been used up by the mother’s body. At this low pO2 the mother's haemoglobin is dissociating with oxygen. For a foetus to obtain enough oxygen from its mother's blood at the placenta, its haemoglobin must have a high affinity for oxygen at a low pO2.
• On an oxygen dissociation graph, the curve for foetal heamoglobin shifts to the left of that for adult haemoglobin. This means that at any given pO2, foetal haemoglobin has a higher percentage saturation than adult haemoglobin.
• After birth, a baby produces adult haemoglobin which gradually replaces foetal haemoglobin. Adult haemoglobin is important for the easy release of oxygen in the respiring tissues of a more metabolically active individual.