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.
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