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].
- 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.