MCAT® ExamBiological and Biochemical Foundations of Living SystemsHard
A physician notes that a patient has an arterial blood pH of 7.25, which is below the normal physiological range. The patient's partial pressure of carbon dioxide (PCO2) is elevated, but their bicarbonate (HCO3-) levels are within the normal range. Which of the following acid-base imbalances is most consistent with these findings?
- ARespiratory alkalosis
- BMetabolic alkalosis
- CRespiratory acidosis
- DMetabolic acidosis
Show answer & explanationAnswer & explanation
Correct answer: C. Respiratory acidosis
The normal arterial blood pH is typically 7.35-7.45. A pH of 7.25 indicates acidosis. Elevated PCO2 suggests a respiratory component, as CO2 forms carbonic acid in the blood. Since bicarbonate (the metabolic component) is normal, the primary cause of the acidosis is the respiratory system. Therefore, this is respiratory acidosis.
Why the other options are wrong
- A. Respiratory alkalosis would involve an increased pH and a decreased PCO2.
- B. Metabolic alkalosis would involve an increased pH and an increased HCO3- level.
- D. Metabolic acidosis would involve a decreased pH and a decreased HCO3- level, with PCO2 potentially normal or decreased as a compensatory mechanism.
Respiratory Acidosis
An acid-base imbalance characterized by a decreased arterial pH and an elevated partial pressure of carbon dioxide (PCO2), typically due to hypoventilation.
- pH < 7.35.
- PCO2 > 45 mmHg.
- Caused by hypoventilation (CO2 retention).
- Kidneys compensate by increasing HCO3- reabsorption.
Memory trick: ROME: Respiratory Opposite, Metabolic Equal.