What is the normal PaCO2 range in a healthy adult?

• A. 35-45
• B. 25-35
• C. 45-55
• D. 15-25

Answer: (A)

PaCO2 in a healthy adult is normally about 35–45 mmHg. This value reflects the balance between CO2 produced by the body's metabolism and CO2 expelled by the lungs through ventilation. The respiratory system adjusts breathing to keep PaCO2 within this narrow window; if ventilation slows, CO2 accumulates and PaCO2 rises above 45, signaling hypoventilation and possible respiratory acidosis. If ventilation speeds up, CO2 is blown off and PaCO2 falls below 35, indicating hyperventilation and possible respiratory alkalosis. At rest and at sea level, the body tends to hold PaCO2 around 40 mmHg, with 35–45 mmHg used as the normal range to accommodate individual variation. Values outside this range point to abnormal ventilation patterns.