The two respiratory disorders
When breathing itself is the cause of an acid–base problem, there are only two possibilities. Too little breathing retains CO2 and the blood turns acidic — respiratory acidosis. Too much breathing washes CO2 out and the blood turns alkaline — respiratory alkalosis. In both, the giveaway is that the PaCO2 moves in the *opposite* direction to the pH.
- Respiratory acidosis: low pH + high PaCO2. Caused by hypoventilation — COPD flares, sedatives, muscle weakness.
- Respiratory alkalosis: high pH + low PaCO2. Caused by hyperventilation — anxiety, pain, fever, altitude, early sepsis.
Compensation: the kidneys help out
The body hates an abnormal pH and fights to restore it. When the lungs cause the problem, the kidneys provide the compensation by adjusting bicarbonate. In a respiratory acidosis the kidneys *retain* bicarbonate to buffer the excess acid; in a respiratory alkalosis they *dump* bicarbonate. The catch is speed: the lungs react in minutes, but the kidneys take roughly one to several days to fully adjust.
Compensation aims to nudge the pH back toward normal, but it rarely overshoots and almost never fully restores it. So a partly compensated gas still shows an abnormal pH — just less abnormal than the raw CO2 alone would predict. This is why the same CO2 can look very different depending on how long it has been present.
Acute vs chronic — the same CO2, two stories
Because the kidneys are slow, the bicarbonate tells you *how long* a respiratory problem has been there. A sudden CO2 rise with a still-normal bicarbonate and a very low pH is acute — the kidneys have not caught up. The same high CO2 with a well-raised bicarbonate and a nearly normal pH is chronic — the kidneys have had time to compensate, as in long-standing COPD.
Same PaCO2, two very different patients
ACUTE respiratory acidosis (minutes-hours):
pH 7.22 PaCO2 60 HCO3- 25
-> CO2 high, bicarbonate still normal,
pH markedly low. Kidneys have NOT compensated.
Think: opioid overdose, acute COPD flare.
CHRONIC respiratory acidosis (days-years):
pH 7.36 PaCO2 60 HCO3- 33
-> Same CO2, but bicarbonate well raised,
pH nearly normal. Kidneys HAVE compensated.
Think: long-standing COPD at baseline.
Key: the bicarbonate, not the CO2, reveals
how long the problem has been present.