In tissue, cellular respiration produces carbon dioxide as a waste product; as one of the primary roles of the cardiovascular system, most of this CO2 is rapidly removed from the tissues by its hydration to bicarbonate ion. The bicarbonate ion present in the blood plasma is transported to the lungs, where it is dehydrated back into CO2 and released during exhalation. These hydration and dehydration conversions of CO2 and H2CO3, which are normally very slow, are facilitated by carbonic anhydrase in both the blood and duodenum. While in the blood, bicarbonate ion serves to neutralize acid introduced to the blood through other metabolic processes ; likewise, any bases are neutralized by carbonic acid.
Regulation
As calculated by the Henderson–Hasselbalch equation, in order to maintain a normal pH of 7.4 in the blood, 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 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.
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 of the acid dissociation constant of carbonic acid. It is equal to 6.1.
is the concentration of bicarbonate in the blood
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:
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 /mmHg
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:
pH is the acidity in the blood
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 rearranging the equation and applying Henry's law, the equation becomes:
where K’ is the dissociation constant from the pKa of carbonic acid, 6.1, which is equal to 800nmol/L. By multiplying K’ and 0.03 and rearranging with respect to HCO, the equation is simplified to:
In other tissues
The bicarbonate buffer system plays a vital role in other tissues as well. In the human stomach and duodenum, the bicarbonate buffer system serves to both neutralize gastric acid and stabilize the intracellular pH of epithelial cells via the secretion of bicarbonate ion into the gastric mucosa. In patients with duodenal ulcers, Helicobacter pylori eradication can restore mucosal bicarbonate secretion, and reduce the risk of ulcer recurrence.