(Assuming temp = 37C, pH = 7.40, Base excess = 0)
The PO2 at which oxygen-carrying proteins is 50% saturated
--> An index of oxygen affinity
--> The lower p50 value, the high O2 affinity
For adult haemoglobin (HbA)
* p50 is at 26.6mmHg
For foetal haemoglobin (HbF)
* p50 is at 18-20mmHg
* Due to lack of 2,3-DPG binding (no beta chain)
NB:
Sigmoid shaped
Two portions:
Fall in alveolar pO2
--> Little change in O2 sats
As peripheral tissues extract O2
--> Drop in PO2 is not too great
--> Maintains O2 diffusion gradient
Positive cooperativity
When one of the 4 globin chains in Hb binds with O2
--> Structural changes in Hb
--> O2 affinity of the haem of the remaining chains increased
--> "Positive cooperativity
--> Sigmoid shaped ODC
Two effects
Carboxyhaemoglobin dissociation curve is extremely left-shifted and is a rectangular hyperbola.