In all active tissues, metabolism generates, as a waste, carbon dioxide (=CO2).
This CO2 has to go back to the lungs to be exhaled. This transport is also performed by the blood flow.
There are three transport systems for CO2:
1. Diffused in plasma (10%)
2. Bound to the hemoglobin molecule (20%)
3. Converted to bicarbonate ions (70%)
CO2, just like O2, is dissolved in the plasma of blood. But because CO2 dissolves much better then oxygen (about 20 x), this amounts too much more CO2 being transported by the plasma than O2 (plasma only carries about 0.5% of the total oxygen).