What is the advantage of having a sigmoid shape dissociation curve for a maternal haemoglobin?
The curve demonstrates that at a normal PaO2 (13kPa), haemoglobin is approaching 100% saturation. The curve has a sigmoid shape owing to the cooperative binding of haemoglobin with oxygen. When one molecule of oxygen binds to the ferrous iron, steric reactions cause movement of the globin polypeptide chains. This facilitates the binding of further oxygen molecules. Thus at low oxygen tensions it is initially difficult for haemoglobin to combine with oxygen but this process gets much easier as more oxygen is available and as more oxygen combines with haemoglobin i.e. at the higher oxygen tensions available in the lungs. This is evident by the steep rise in the curve, until it plateaus as it approaches 100% saturation. The steep section of the curve is extremely important as it demonstrates that below 92% saturation the drop in saturated haemoglobin falls very quickly per unit partial pressure of oxygen.
