Explain acid-base disorders with metabolic vs respiratory disturbances and the compensatory mechanisms involved.

• A. Respiratory disorders primarily involve PaCO2; metabolic disorders involve HCO3-; compensation occurs via renal or respiratory adjustments: kidneys adjust bicarbonate in metabolic disturbances, lungs adjust CO2 in respiratory disturbances; evaluation uses pH, PaCO2, and HCO3-.
• B. In acid-base disorders, pH is not important; compensation is not relevant.
• C. Metabolic disorders primarily involve PaCO2; respiratory disorders involve HCO3-; compensation via renal or respiratory adjustments.
• D. Metabolic and respiratory disturbances are diagnosed by irrelevant labs; no compensation.

Answer: (D)

Acid-base balance hinges on two main drivers: bicarbonate (metabolic) and carbon dioxide (respiratory). The pH tells you whether the blood is acid or alkali, but to identify the cause you look at PaCO2 and HCO3-. If the primary abnormality is bicarbonate, you’re dealing with a metabolic disturbance; if the primary abnormality is PaCO2, you’re dealing with a respiratory disturbance.

In metabolic disturbances, the body tries to correct pH by changing ventilation. For metabolic acidosis, CO2 is blown off to raise pH (hyperventilation), so PaCO2 falls while HCO3- is low. For metabolic alkalosis, hypoventilation raises PaCO2 to bring pH back toward normal, and HCO3- is high. The lungs act quickly, within minutes to hours, to provide this compensation.

In respiratory disturbances, the kidneys compensate by adjusting bicarbonate handling. In respiratory acidosis (CO2 retention, low pH), the kidneys increase HCO3- reabsorption and hydrogen ion excretion to raise pH, a process that develops over hours to days. In respiratory alkalosis (low CO2, high pH), the kidneys reduce HCO3- reabsorption (and increase bicarbonate excretion) to lower pH, with renal changes taking effect more slowly.

Evaluation centers on pH, PaCO2, and HCO3- to identify the primary disorder and whether the compensatory response is appropriate. Recognize that mixed disorders can yield a near-normal pH with discordant PaCO2 and HCO3- values, requiring careful assessment.