<P> As calculated by the Henderson - Hasselbalch equation, in order to maintain a normal pH of 7.4 in the blood (whereby the pKa of carbonic acid is 6.1 at physiological temperature), a 20: 1 bicarbonate to carbonic acid must constantly be maintained; this homeostasis is mainly mediated by pH sensors in the medulla oblongata of the brain and probably in the kidneys, linked via negative feedback loops to effectors in the respiratory and renal systems . 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 CO . By Le Chȃtelier's Principle, the release of CO from the lungs pushes the reaction above to the left, causing carbonic anhydrase to form CO 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 excreting H ions into the urine while, at the same time, secreting HCO ions into the blood plasma, or vice versa, depending on whether the plasma pH is falling or rising, respectively . </P> <P> A modified version of the Henderson--Hasselbalch equation can be used to relate the pH of blood to constituents of the bicarbonate buffer system: </P> <Dl> <Dd> p H = p K a H 2 C O 3 + log ⁡ ((H C O 3 −) (H 2 C O 3)) (\ displaystyle pH = pK_ (a ~ H_ (2) CO_ (3)) + \ log \ left ((\ frac ((HCO_ (3) ^ (-))) ((H_ (2) CO_ (3)))) \ right)) </Dd> </Dl> <Dd> p H = p K a H 2 C O 3 + log ⁡ ((H C O 3 −) (H 2 C O 3)) (\ displaystyle pH = pK_ (a ~ H_ (2) CO_ (3)) + \ log \ left ((\ frac ((HCO_ (3) ^ (-))) ((H_ (2) CO_ (3)))) \ right)) </Dd>

In the bicarbonate buffering system in the body bicarbonate acts as a