Erythrocytes (red blood cells) in the mammalian body are filled with a globular protein called Haemoglobin. This molecule consists of 4 polypeptide chains each with an Iron Haem group in the centre. This Haem group is very important for gaseous exchange.
Once the Erythrocyte, laden with Oxygen molecules, reaches some tissues which have an Oxygen deficit, the following reaction occurs:
CO2 + H2O <> H2CO3 <> HCO3- + H +
(<> = a reversible reaction)
This equation is catalysed by the highly efficient enzyme called Carbonic Anhydrase. The CO2 is absorbed into the red blood cell then in this equation it is reacted with water to make firstly to Hydrogen Carbonate and then to Hydrogen Carbonate- ions and H+ ions. The HCO3- ions are pumped out of the cell and are replaced with Cl- ions to keep the charges balanced.
The next stage in this process is as follows:
H+ + HbO8 <> HHb+ + 4O2
Here I have used Hb for Haemoglobin for simplicity because Haemoglobin is actually many thousands of molecules long.
This stage of the reaction involves the Hydrogen ions bonding with HbO8 (Oxygen bonded with Haemoglobin). The Hydrogen ions displace the Oxygen molecules from the Haem groups and allow the Oxygen molecules to come into solution in the red blood cell. These then diffuse through the phospholipid bilayer membrane of the red blood cell and move into the surrounding tissues where Oxygen is needed leaving the Hydrogen bonded with the Haemoglobin, acting as a spectator ion, as it is not actually used for anything.
Once this reaction has occurred there will be Oxygen in the tissues and the Carbon Dioxide which was in the tissues will now be carried back to the lungs to be exchanged for more Oxygen.
This reaction is entirely reversible and the opposite reaction happens at the lung to absorb new Oxygen.