CO2 is a waste product of aerobic metabolism. The body must be able to eliminate this waste product to maintain normal function. CO2 dissolves out of the tissue cells into the blood stream where it is carried to the lungs for elimination through ventilation.
Fig. 7-1. How CO2 is converted to HCO3 at the tissue sites. Most of the CO2 that is produced at the tissue cells is carried to the lungs in the form of HCO3.
Fig. 7-2. How HCO3 is transformed back into CO2 and eliminated in the alveoli.
Dissolved in solution
CO2 dissolves into the plasma and the intracellular fluid of the erythrocyte. The partial pressure exerted by the CO2 in solution is what drives the rest of the reactions, so even though the dissolved component is only responsible for approximately 5% of the CO2 that is released to the lungs, it is still an important transport role.
For every mm Hg PCO2 pressure, 0.03 mEq of CO2 are physically dissolved in one liter of plasma. The normal arterial PCO2 is 40 mm Hg, therefore the amount of CO2 dissolved in the plasma can be calculated as follows:
Be careful not to confuse the factor for quantifying dissolved CO2 (0.03) with the factor for quantifying the dissolved O2 (0.003)!
Combined with proteins
CO2 combines with proteins in the plasma and forms carbamino compounds, and combines with hemoglobin in the RBC to form carbaminohemoglobin.
Converted to Bicarbonate
In the plasma at the tissue / systemic capillary level, the hydrolysis (the combining of CO2 with H2O) is a very slow reaction, so only a small amount forms carbonic acid which rapidly dissociates into H+ and HCO3- ions. However, in the RBC there is a catalyst called carbonic anhydrase that accelerates the hydrolysis (13,000 times more rapidly), so that the majority of the CO2 in the RBC is converted quickly to carbonic acid which then dissociates into the hydrogen and bicarbonate ions. Bicarbonate is exchanged for the chloride ion in the plasma, and the reduced hemoglobin binds with the hydrogen ion. At the alveoli / pulmonary capillary level the reversal of the pressure gradients causes all of these processes to reverse so that CO2 diffuses out into the lungs.
Fig. 7-3. Carbon dioxide dissociation curve.
Fig. 7-4. Carbon dioxide dissociation curve. An increase in the PCO2 from 40 mm Hg to 46 mm Hg raise the CO2 content by about 5 vol.%. PCO2 changes have a greater effect on CO2 content levels than PO2 changes on O2 levels.
Fig. 7-5. Carbon dioxide dissociation curve at two different oxygen/hemoglobin saturation levels (SaO2 of 97% and 75%). When the saturation of O2 increases in the blood, the CO2 content decreases at any given PCO2. This is known as the Haldane effect.
The low SaO2 at the tissue increases the bloods capacity to hold CO2 and facilitates the loading of CO2 into the blood at the tissues; the high SaO2 at the lungs decreases the bloods capacity to hold CO2 and this facilitates its unloading at the lungs.
Fig. 7-6. Comparison of the oxygen and carbon dioxide dissociation curves in terms of partial pressure, content, and shape.