Expansion
Slow but powerful, taking days but capable of large adjustments
Expansion
| Respiratory compensation | Renal compensation | |
|---|---|---|
| Responds to | Metabolic disorders | Respiratory disorders |
| Speed | Minutes to hours | Days, 3 to 5 for completion |
| Mechanism | Change in alveolar ventilation | Bicarbonate handling and acid excretion |
| Capacity | Limited | Large |
Renal mechanisms in acidosis:
- Increased hydrogen ion secretion and bicarbonate reabsorption
- Greatly increased ammoniagenesis from glutamine, which can rise several-fold over days and is the main adaptive component
- Increased titratable acid excretion
In alkalosis: bicarbonate is excreted, giving an alkaline urine.
Neither system fully corrects the pH, since complete correction would remove the stimulus for compensation. A pH that has returned entirely to normal therefore suggests either a mixed disorder or a chronic, fully compensated respiratory state.
One important exception: in hypovolaemia, the kidney prioritises volume over pH and retains bicarbonate along with sodium, which is how a contraction alkalosis is maintained despite the alkalaemia.