compensation

Compensation is the body's way of fighting back against a shift in blood pH. When one system pushes the blood too acidic or too alkaline, the other system pushes in the opposite direction to limit the damage, like a tightrope walker leaning one way to counter a tilt the other way.

There are two compensating systems. The lungs respond fast, within minutes, by changing how much carbon dioxide they blow off; this is the respiratory compensation for a metabolic problem. The kidneys respond slowly, over hours to days, by retaining or excreting bicarbonate and acid; this is the renal compensation for a respiratory problem. Each disturbance is offset by whichever system is not the cause.

Crucially, compensation softens the pH change but does not usually correct it fully, and it never overshoots to the opposite side. A blood gas showing carbon dioxide and bicarbonate moving in the same direction, with pH only mildly abnormal, points to a single primary disorder with compensation; values that do not fit this pattern suggest a second, mixed disorder.