In the blood of most animals, the bicarbonate buffer system is coupled to the lungs via respiratory compensation, the process by which the rate of breathing changes to compensate for changes in the blood concentration of CO2. By Le Chatelier's principle, the release of CO2 from the lungs pushes the reaction above to the left, causing carbonic anhydrase to form CO2 until all excess acid is 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. 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:  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:  Taken together, the following equation can be used to relate the pH of blood to the concentration of bicarbonate and the partial pressure of carbon dioxide:  where:  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:  formula_5  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 rearranging the equation and applying Henry's law, the equation becomes: formula_6  where "K’" is the dissociation constant from the p"Ka" of carbonic acid, 6.1, which is equal to 800nmol/L (since "K’" = 10−p"Ka" = 10−(6.1) ≈ 8.00X10−07mol/L = 800nmol/L). By multiplying "K’" (expressed as nmol/L) and 0.03 (800 X 0.03 = 24) and rearranging with respect to HCO, the equation is simplified to:  formula_7  The bicarbonate buffer system plays a vital role in other tissues as well.