An excess of acid is called acidosis and an excess in bases is called alkalosis. The process that causes the imbalance is classified based on the etiology of the disturbance (respiratory or metabolic) and the direction of change in pH (acidosis or alkalosis). This yields the following four basic processes:
| process | pH | carbon dioxide | compensation |
| metabolic acidosis | down | down | respiratory |
| respiratory acidosis | down | up | renal |
| metabolic alkalosis | up | up | respiratory |
| respiratory alkalosis | up | down | renal |
