Expansion
pH depends on the ratio of bicarbonate to carbon dioxide, not their absolute values
Expansion
pH = pKa + log ([HCO3-] / (0.03 x PaCO2))
With a pKa of 6.1 and normal values of bicarbonate 24 mmol/l and PaCO2 40 mmHg, the ratio is 20:1, giving a pH of 7.4.
Two things follow immediately:
- Only the ratio matters. Doubling both bicarbonate and carbon dioxide leaves the pH unchanged, which is exactly what compensation achieves
- The kidney controls the numerator (bicarbonate, the metabolic component) and the lung controls the denominator (carbon dioxide, the respiratory component)
The bicarbonate system seems a poor buffer, since its pKa of 6.1 is far from physiological pH. It works so well because it is an open system: carbon dioxide is continuously removed by the lungs, so the reaction is pulled away from acid, giving it far greater capacity than its pKa suggests.
Other buffers matter too: haemoglobin is the most important in blood, and bone and protein provide slow, large-capacity buffering in chronic acidosis.