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In all active tissues, metabolism generates, as a waste, carbon dioxide (=CO2).
There are three transport systems for CO2:
It is also possible for CO2 to bind to the hemoglobin molecule.
In this case, the CO2 is bound to the globin part of the hemoglobin and not to the heme part. So, it does not interfere with the oxygen transport!
In fact, it is slightly easier for hemoglobin to bind with CO2 if there is less binding of that same hemoglobin with oxygen.
This is very nice! As erythrocytes flow towards active tissue, the oxygen content of the hemoglobin molecules decreases (as it gives oxygen to the tissue).
This ‘release’ of oxygen makes the hemoglobin molecule more sensitive to CO2.
This is the main transport route for CO2 , which takes care of 70% of the total transport of CO2 .
This is a chemical reaction that binds CO2 with water (H2O) to create carbonic acid:
In principle, this reaction can take place anywhere in the body, as long as there is water. But the reaction is veryslow.
There is however an enzyme that increases the speed of this reaction. This enzyme is carbonic anhydrase and, this is the important part, this enzyme is only available in the erythrocytes!
So, the CO2 has to diffuse into the erythrocyte to be bound to water.
The next important thing is that carbonic acid (=H2CO3) is not stable but dissociates (= splits) into one hydrogen ion (=H+) and one bicarbonate ion (=HCO3–).
So, in summary, the CO2 molecule we started with, is converted into a bicarbonate ion.
The bicarbonate ions (together with the H+ ions) can diffuse into the plasma (where they play an important role in the buffer system; see later).
Note that the reaction from CO2 to bicarbonate is reversible. It may also go the other way.
This is of course what happens in the lungs; the bicarbonate ions are converted back, in the erythrocytes, to CO2 to be exhaled in the air.
There is however one small problem. Both H+ and HCO3– are ions and have a charge.
The H+ ion is bound to hemoglobin molecule, thereby neutralizing its charge. This is actually the basis for the Bohr effect.
But the negative bicarbonate ions diffuse out of the erythrocytes into the plasma (concentration effect; there is more bicarbonate inside than outside the red blood cell).
But this would make the erythrocyte more positive. To compensate for this charge, other (negative) ions will shiftto inside the cell; the most common negative ion is the chloride ion (=Cl–).
Therefore, this compensation system is called the Chloride-shift.
