Introduction
Compensatory mechanisms restore pH towards normal by altering pCO2 and HCO3–.
Compensation of metabolic acidosis [2-4]
The lowered pH acts on peripheral chemoreceptors to stimulate the ventilation. Respiratory rate increases and pCO2 falls:
| ↓CO2 + H2O ← H2CO3 ← H+ + HCO3– |
[H+] therefore falls, as does [HCO3–]. There is also increased reabsorption of HCO3– and increased excretion of H+ from the kidneys but this is not instant.
Compensation of metabolic alkalosis [2,3,5]
Conversely, in metabolic alkalosis, the higher pH acts on the chemoreceptors to reduce ventilation and increase pCO2:
| ↑CO2 + H2O ← H2CO3 ← H+ + HCO3– |
The renal response is then to decrease HCO3– reabsorption and decrease H+ excretion. This usually occurs fairly quickly, but if the alkalosis is caused by vomiting, resulting in dehydration, the overriding renal response is to increase Na+ and HCO3– reabsorption. Therefore, effective rehydration will help to more rapidly correct the alkalosis.
Compensation of respiratory acidosis [2,3,6]
The problem here is within the ventilatory system, with the kidneys acting to compensate which can take a significant length of time (up to two days). The [H+] is raised, thus the rate of H+ secretion is also increased. This results in increased HCO3– reabsorption, despite HCO3– already being higher as a result of the equation shifting to the right:
| ↑CO2 + H2O → H2CO3 → ↑H+ + ↑HCO3– |
Although the secretion of H+ brings the pH closer to normal, the pH will not be restored to normal without correction of the underlying respiratory disorder.
Compensation of respiratory alkalosis [2,3]
In respiratory alkalosis, the [H+] decreases due to a primary reduction in pCO2. There is therefore less H+ in the renal tubules and reduced H+ secretion. As a consequence, less HCO3– is reabsorbed causing a further fall in [HCO3–]. To restore pCO2 and HCO3– completely to normal, the primary ventilatory problem must be corrected (i.e. The respiratory rate must reduce).
Learning bite
The compensatory response of CO2 or bicarbonate is in the same direction as the initial abnormal change of CO2 or bicarbonate.