Variation in Haemoglobin
Red blood cells contain Hb. Hb is a large protein with a quaternany structure. Its made up of 4 polypeptide chains. Each chain has a haem group which contains iron and makes blood red. Hb has a high affinity for oxygen and each molecule can carry 4 oxygen molecules.
In the lungs oxygen joins to hameoglobin forming oxyhaemoglobin, this is a reversible reaction.
The pO is a measure of oxygen concentraiton. Vice cersa with pCO . Oxgyen loads where there is a high pO (alveoli) and unloads where there's a low pO (respiring tissues).
A dissociation curve shows how affinity for oxygen varies, how saturated the Hb is with oxygen and a given pO. The graph is S shaped because when Hb combines with the first molecule of oxygen its shape alters in a way that makes it easier for other molecules to join. However as the Hb starts to become more saturated it gets harder for molecules to join. So the curve has a steep bit in the middle where its easy to join and shallow at each end where its harder.
More Variation in Haemoglobin
Hb gives up its oxygen more readily at a higher pCO . When cells respire they produce carbon dioxide so Hb unloads oxygen more. The curve shifts down, lower saturation this is the BOHR effect.
Hb is different in different organisms. Organisms that live in an environment with a low concentration of oxgen have Hb with a higher affinity for oxygen, shifts to the left. Organisms that are very active have Hb with a lower affinity for oxygen, shifts to the right.