Foetal Haemoglobin

A developing foetus obtains oxygen not from its own lungs, but from its mother's blood. In the placenta, the mother's blood is brought very close to that of the foetus, allowing diffusion of various substances from mother to foetus or vice versa.

Oxygen arrives at the placenta in combination with haemoglobin, inside the mother's red blood cells. The partial pressure of oxygen in the blood vessels in the placenta is relatively low, because the foetus is respiring. The mother's haemoglobin therefore releases some of its oxygen, which diffuses from her blood into the foetal blood.

The partial pressure of oxygen in the foetal blood is only a little lower than that in its mother's blood. However, the foetal and maternal haemoglobin have different structures: foetal haemoglobin combines more readily with oxygen than does maternal, so the foetal haemoglobin will thus 'pick up' oxygen which the maternal haemoglobin has 'dropped'. Foetal haemoglobin is said to have a higher affinity for oxygen than adult haemoglobin.

Acoordingly, a dissociation curve for foetal haemoglobin shows that, at each partial pressure of oxygen, foetal haemoglobin is slightly more saturated than adult haemoglobin.