Which of the following characterizes respiratory acidosis?

• A. Excess of bicarbonate
• B. Deficit of bicarbonate
• C. Excess of dissolved carbon dioxide (PCO2)
• D. Deficit of dissolved carbon dioxide (PCO2)

Answer: (C)

Respiratory acidosis occurs when ventilation is insufficient to remove the CO2 produced by the body, so carbon dioxide builds up in the blood. That excess CO2 shifts the chemical balance of the bicarbonate buffer system toward carbonic acid, which dissociates to hydrogen ions and water, lowering the blood’s pH. The hallmark feature is increased dissolved carbon dioxide, reflected as a higher PCO2. Bicarbonate may stay normal at first, but over time the kidneys can compensate by retaining more bicarbonate to buffer the increased H+, partially mitigating the pH drop. So, the description of excess dissolved carbon dioxide best captures the core change in respiratory acidosis.