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One ratio, two organs
pH follows the ratio of HCO3 to PaCO2 (Henderson-Hasselbalch). The kidneys own the numerator on a scale of days; the lungs own the denominator on a scale of minutes. Every disorder and every compensation is one of those two organs moving its own term, and compensation is always in the same direction as the primary change, never enough to normalize the pH.
The gap is a window
The anion gap (Na − Cl − HCO3) exposes unmeasured anions. A metabolic acidosis with a normal gap lost bicarbonate (gut or kidney); with a high gap it gained acid (lactate, ketoacids, toxins, uremia). Albumin is the main normal unmeasured anion, so hypoalbuminemia shrinks the expected gap: correct for it before declaring normality.
Compensation obeys arithmetic
Winter's formula for metabolic acidosis, 0.7 per mmol rules for alkalosis, 1-versus-3.5 and 2-versus-5 per 10 mmHg for acute and chronic respiratory disorders: these expected values turn 'compensated' from an impression into a checked calculation, and their violation is how mixed disorders are found.
Pearls
- Perfect compensation still leaves pH abnormal; a normal pH with abnormal partners is two disorders.
- The delta ratio asks whether the HCO3 fall matches the gap rise; mismatch means a hidden second process.
Connected