Fig. 1

Physiology of CO₂ production and transport. In cells, CO₂ is produced (in mitochondria) as a byproduct of substrate oxidation. Under anaerobic conditions, CO₂ is generated in small amounts, as the results of HCO₃⁻ buffering of protons released by lactic acid and the hydrolysis of ATP. CO₂ diffuses into the interstitial tissues and then into capillaries, where it is transported as dissolved CO₂ in plasma (in equilibrium with the PCO₂), bound to hemoglobin as carbamino-hemoglobin (HbCO₂) in red blood cells (RBC), and as HCO₃⁻, following the reaction of CO₂ with H₂O within RBC, a reaction catalyzed by carbonic anhydrase to form HCO₃⁻ and H⁺. HCO₃⁻ exits the RBC in exchange with chloride anions (Cl⁻), whereas protons are buffered by hemoglobin, forming HbH