Buffer Systems
Chemical buffers (seconds): bicarbonate-carbonic acid (most important, 50%), phosphate, hemoglobin, proteins. Immediate but limited. Respiratory buffers (minutes to hours): lungs adjust CO2 by changing ventilation rate. Renal buffers (hours to days): kidneys adjust HCO3 reabsorption/excretion and H+ secretion.
Respiratory Compensation
For metabolic acidosis: hyperventilation (Kussmaul respirations in DKA) to blow off CO2. For metabolic alkalosis: hypoventilation to retain CO2. Limited: cannot overcompensate (pH will not go past 7.40). Fast: begins within minutes, maximal in 12-24 hours. Expected PaCO2 (Winter): 1.5 x HCO3 + 8 (+/-2).
Renal Compensation
For respiratory acidosis: kidneys retain HCO3 and excrete H+. For respiratory alkalosis: kidneys excrete HCO3 and retain H+. Slow: begins in hours, maximal in 2-3 days. Acute (sudden change): HCO3 changes 1 mEq/L per 10 mmHg PaCO2 (acidosis) or 2 mEq/L (alkalosis). Chronic (2-3 days): 4 mEq/L (acidosis) or 5 mEq/L (alkalosis).
Important Notes
- The body never overcompensates — if pH is on the other side of 7.40, there is a mixed disorder.
- Winter formula: expected PaCO2 = 1.5 x HCO3 + 8. If actual is significantly different, there is a mixed disorder.
Clinical Tips
- Respiratory compensation is fast (minutes), renal is slow (days) — chronic respiratory disorders show more renal compensation.
- Kussmaul respirations (deep, rapid) are respiratory compensation for metabolic acidosis (DKA).
Summary
Compensatory mechanisms (chemical, respiratory, renal) work at different speeds to maintain pH — understanding expected compensation helps identify mixed disorders.
