The balanced equation for the formation of carbonic acid is CO2 + H2O <-> H2CO2 <-> H+ + HCO3-. Carbonic acid forms from the dissolution of water, or H2O, in carbon dioxide, or CO2.
Carbonic acid, or H2CO3, is a weak acid that plays a vital role in breathing, maintaining the normal range of pH in the blood, global warming, and carbonation of drinks. In a liquid solution, carbonic acid readily disassociates into the charged bicarbonate ion (HCO3-) and a proton (H+). Once CO2 forms during metabolic processes in cells, it converts into bicarbonate, an ion. This conversion enables its transportation in blood into the lungs, where the bicarbonate converts back to CO2 for exhalation.
In mammals, the same process happens by carbonic anhydrase, an enzyme that can speed up the reaction back and forth between the production of carbonic acid and its conversion back to carbon dioxide and water. Bicarbonate is used in medical settings as well, including as a temporary measure in cardiopulmonary resuscitation to attempt to counteract acidosis or reversal of overdose in tricyclic antidepressants, due to its ability to buffer the pH of the blood.
If there is too much acid (H+) in the solution, the equilibrium shifts to the conversion of bicarbonate (HCO3-) to carbonic acid (H2CO3) which can then form CO2 + H2O. This process decreases the amount of acidity in the blood. While hyperventilating, people exhale excessive amounts of CO2, and the equation shifts to the production of CO2 and utilization of the acid in the blood, thereby causing respiratory alkalosis. Failure to breathe off CO2 adequately, such as in obstructive sleep apnea, changes the equilibrium to the production of carbonic acid, and ultimately acidifies the blood.