I would have to go back to my medical books for a definitive answer. But, off the top of my head: CO2 (Carbon Dioxide) is produced by muscles at work (its what the lungs exchange for oxygen when you breath). As you exercise, the CO2 builds and combines with water (H2O) to produce Carbolic Acid which is unstable and generates bicarbonate (HCO3 plus the extra H).

The body however is both very finicky about maintaining its acid/base balance and very capable of maintaining that balance.

The body has various ways of maintaining that balance -- one of which (the primary method) is respiration. As you breath, the lungs expel the CO2 -- which decreases the whole reaction. And so as you exercise you breath harder to expel the CO2 and return the body to equilibrium. Surprisingly to most people, the body triggers the pulmonary system to breath harder NOT because of a lack of oxygen but rather because of an excess of CO2! That is, the body does not have a "pulse ox" meter -- it uses a CO2 meter instead to determine how hard you will breath!

If respiration fails to return the body to its preferred equilibrium state, it then goes on to other, more extreme measures of maintaining that balance. In the end, the body either maintains its acid/base balance or it dies -- so it takes that chore very seriously and has multiple methodologies always in readiness to maintain that balance.