the reaction above to the left, causing carbonic anhydrase to form CO2 until all excess protons are removed. Bicarbonate concentration is also further regulated by renal compensation, the process by which the kidneys regulate the concentration of bicarbonate ions by secreting H+ ions into the urine while, at the same time, reabsorbing HCO ions into the blood plasma, or vice versa, depending on whether the plasma pH is falling or rising, respectively. Henderson–Hasselbalch equation A modified version of the Henderson–Hasselbalch equation can be used to relate the pH of blood to constituents of the bicarbonate buffer system: where: pKa H2CO3 is the negative logarithm (base 10) of the acid dissociation constant of carbonic acid. It is equal to 6.1. [HCO] is the concentration of bicarbonate in the blood [H2CO3] is the concentration of carbonic acid in the blood When describing arterial blood gas, the Henderson–Hasselbalch equation is usually quoted in terms of pCO2, the partial pressure of carbon dioxide, rather than H2CO3. However, these quantities are related by the equation: where: [H2CO3] is the concentration of carbonic acid in the blood kH CO2 is a constant including the solubility of carbon dioxide in blood. kH CO2 is approximately 0.03 (mmol/L)/mmHg pCO2 is the partial pressure of carbon dioxide in the blood Combining these equations results in the following equation relating the pH of blood to the concentration of bicarbonate and the partial pressure of carbon dioxide: where: pH is the acidity in the blood [HCO] is the concentration of bicarbonate in the blood, in mmol/L pCO2 is the partial pressure of carbon dioxide in the blood, in mmHg Derivation of the Kassirer–Bleich approximation The Henderson–Hasselbalch equation, which is derived from the law of mass action, can be modified with respect to the bicarbonate buffer system to yield a simpler equation that provides a quick approximation of the H+ or HCO concentration without the need to calculate logarithms: Since the partial pressure of carbon dioxide is much easier to obtain from measurement than carbonic acid, the Henry's law solubility constant – which relates the partial pressure of a gas to its solubility – for CO2 in plasma is used in lieu of the carbonic acid concentration. After solving for H+ and applying Henry's law, the equation becomes: where K’ is the dissociation constant of carbonic acid, which is equal to 800 nmol/L (since K’ = 10−pKaH2CO3 = 10−(6.1) ≈ 8.00×10−7 mol/L