Acid-base Disturbances

Acid-base disturbances can be of either:

  • Respiratory origin where the disturbance is primarily of CO2 exchange
  • Metabolic origin where the disturbance is due to bicarbonate

The body has a number of defences to these changes but these only minimise the pH disturbance. The pH moves towards normal – this is termed compensation. Correction is when normal pH is restored [3].

  1. The first defence is by buffering as outlined previously. In the extracellular fluid, this occurs almost immediately; intracellular buffering is a little slower. The next compensatory change in pH occurs by altering respiratory rate and therefore blood pCO2. An increase in CO2 reduces the pH:

CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3

↑CO2 shifts the equation to the right → H2CO3 → ↑ H+ i.e. ↓pH
↓CO2 shifts the equation to the left → ↓ H+  i.e. ↑pH


The final compensatory change is renal handling (i.e. excretion) of acid (and subsequent reabsorption of HCO3) as described previously [2].

Learning bite

The body employs various mechanisms to minimise disturbances in acid-base balance – this is called compensation.