When haemoglobin is fully saturated with oxygen it is called oxygenated haemoglobin (or oxyhaemoglobin) and when deoxygenated it is called deoxyhaemoglobin.
When deoxyhaemoglobin is exposed to oxygen, the uptake of each molecule of oxygen facilitates the uptake of the remainder (a property referred to as the quaternary function of haemoglobin). This process occurs in less than 0.01 seconds and is called association. The opposite of this, i.e. the off-loading of oxygen, is known as dissociation.
It is the presence of haemoglobin that permits sufficient O2 to be transported to tissues. Due to the high PO2 at the alveolus there is ready transfer of O2 to the RBC across the pressure gradient. Once all the haemoglobin molecules have taken up four oxygen molecules, the haemoglobin is said to be 100% saturated.
Saturated haemoglobin
This is often expressed as SaO2 = 100%, where   S = saturation and a = arterial.
When the SaO2 = 100%, each gram of haemoglobin in the arterial system is carrying 1.34 ml oxygen – a figure known as the oxygen carrying capacity of haemoglobin (OCC Hb).
Consequently each litre of arterial blood, containing 150 g haemoglobin, will carry 150 x 1.34 = 201 ml oxygen. Compare this to the 3 ml of oxygen which would be carried if the oxygen was merely dissolved in the blood.
Learning bite
The presence of haemoglobin allows nearly seventy times more oxygen to be carried than would be possible if blood relied simply on oxygen dissolved in the plasma.