Oxygen dissociation curves show the relationship between oxygen levels (as partial pressure) and haemoglobin saturation

• The partial pressure of oxygen describes the pressure exerted by oxygen within a mixture of gases (it reflects oxygen concentration)
  

The oxygen dissociation curve for adult haemoglobin is sigmoidal (i.e. S-shaped) due to cooperative binding

• At high pO2 levels (in the lungs), there is very little dissociation of oxygen from haemoglobin (curve plateaus)

• When pO2 levels are moderate (in respiring tissues), small changes in pO2 can cause large changes in percentage saturation (steep gradient)

• At extremely low pO2 levels (in hypoxic tissues), the haemoglobin will not readily remove oxygen from the tissue (the curve is relatively flat)
  

As foetal haemoglobin has a higher affinity for oxygen than adult haemoglobin, the dissociation curve will be shifted to the left

• This means that within the placenta, foetal haemoglobin will load oxygen when the adult haemoglobin is unloaded