We produce the energy ATP molecules. Now, these ATP molecules are used for muscle contraction, for the actin contraction. But the CO2 molecules, a byproduct, are produced and these cannot be used in any useful way. And so the CO2 molecules are expelled from the cell and they enter the blood plasma of the capillary, the nearby capillary. Now, because carbon dioxide is nonpolar, only a very small portion, about 5%, will actually remain dissolved in the blood plasma. The majority of it, the rest of it will enter the cytoplasm of the red blood cell.