BicarbonatCO2System

The bicarbonate-CO2 system, also known as the bicarbonate buffering system, is the principal extracellular buffer of pH in physiological fluids, especially blood. It consists of carbonic acid (H2CO3) formed from carbon dioxide (CO2) and water (H2O), which rapidly equilibrates to bicarbonate (HCO3−) and hydrogen ions (H+). The reaction is catalyzed by carbonic anhydrase, mainly in red blood cells and renal tubules.

In blood, CO2 is transported in three main forms: dissolved CO2, bicarbonate, and carbamino compounds. In erythrocytes, CO2 hydration to H2CO3 is accelerated by carbonic anhydrase; HCO3− is exchanged for Cl− across the RBC membrane (the chloride shift) and carried in plasma, facilitating the transport of CO2 to the lungs.

The system buffers pH via the Henderson-Hasselbalch relationship. The pH of blood is determined by the ratio of [HCO3−] to dissolved CO2; pH = pKa + log([HCO3−]/[CO2(aq)]). The pKa of carbonic acid in blood is about 6.1 for the carbonic acid/bicarbonate buffer at body conditions, and the effective pH depends on CO2 levels and hemoglobin’s buffering capacity.

Regulation occurs through two main mechanisms. The respiratory system modulates CO2 by adjusting ventilation: increased ventilation lowers CO2 and raises pH, while decreased ventilation raises CO2 and lowers pH. The kidneys regulate bicarbonate by reabsorbing it and generating new bicarbonate, increasing buffering capacity over days.

Clinical relevance: Disturbances in this system contribute to metabolic or respiratory acidosis and alkalosis, with compensatory responses. Measurement of blood gases and bicarbonate levels is a standard tool for assessing acid-base status.