Which ABG component reflects the respiratory component of acid-base status?

• A. pH
• B. PaO2
• C. HCO3-
• D. PaCO2

Answer: (D)

PaCO2 is used to gauge the respiratory part of acid-base balance because it’s the gas the lungs actively regulate. CO2, when dissolved in blood, forms carbonic acid and influences pH directly. Ventilation adjusts PaCO2: increasing it (hypoventilation) raises acidity and lowers pH—respiratory acidosis; decreasing it (hyperventilation) lowers acidity and raises pH—respiratory alkalosis. The bicarbonate (HCO3-) piece of the equation is handled by the kidneys and represents the metabolic component, helping compensate for respiratory disturbances. The overall pH reflects the combined influence of both, while PaO2 simply shows oxygenation status and isn’t a direct measure of acid-base balance. So the component that best reflects the respiratory side is the arterial CO2 level, PaCO2.