Because if one negative charge exits the cell, we have to basically take a negatively charged atom and make it go into the cell to maintain an electrically neutral state, because we do not actually want to affect the electrochemical gradient that exists between the inside of the red blood cell and the outside the blood plasma. So we essentially exchange a single bicarbonate ion for a single chloride ion. And this is what we call the chloride shift. So the chloride shift takes place to maintain a balance of electrical charge between the inside of the red blood cell, the cytoplasm portion, and the outside environment, the blood plasma. So about 90% of the carbon dioxide exists in its bicarbonate ion state inside the blood plasma. About 5% exist dissolved inside our blood plasma at CO2.