What is the relationship between pH and PaCO2 in respiratory alkalosis?

• A. pH increases while PaCO2 decreases
• B. pH decreases while PaCO2 decreases
• C. pH increases while PaCO2 increases
• D. pH decreases while PaCO2 increases

Answer: (A)

In respiratory alkalosis the main change is too little CO2 in the blood because of increased ventilation. CO2 contributes to acidity, so when PaCO2 falls, hydrogen ion concentration drops and the pH rises. Put simply: pH goes up as PaCO2 goes down. This inverse relationship is reflected in the Henderson–Hasselbalch framework, where a decrease in PaCO2 increases the pH. Early on, bicarbonate is near normal, and renal compensation (which lowers bicarbonate) may occur later to partially counteract the alkalemia.