The majority of the carbon dioxide transported through the human bloodstream is in the form of the bicarbonate anion (HCO3-), and it is carried within the red blood cells. A much smaller portion of the bicarbonate anion is carried by the blood plasma. Carbon dioxide in the form of bicarbonate can account for percentages as high as 70 to 90 percent of the blood's overall CO2 transport.
Carbon dioxide, which is about 20 times more soluble in blood than oxygen, hydrolyzes to produce carbonic acid (H2CO3), and then quickly disassociates into its ionic components of HCO3- and H+. The bicarbonate anion also plays an important role in the regulation of the blood's pH level.
A considerably small portion of the carbon dioxide in the bloodstream, about 1 percent or less, is transported in the form of a carbamino compound in the blood plasma. Carbamino-Hb, which is formed by carbon dioxide combining with hemoglobin within the red blood cells, accounts for about 21 percent of the transport coming from tissue cells. About 10 percent of the carbon dioxide in the bloodstream is dissolved directly by the plasma and the intracellular fluid of red blood cells. The normal rate of production of carbon dioxide by tissue cell metabolism while the body is at rest is about 200 milliliters per minute.