Acid-Base Balance -- Answers

1. A. Systemic arterial blood has a higher (more alkaline) pH than systemic venous blood because of the CO2 added by metabolizing cells as blood passes through the systemic vascular beds. Urine is generally more acid than plasma because of the necessity of excreting the excess fixed (non-volatile) acids created by metabolism.

2. B.

3. B. Respiratory compensation for metabolic alkalosis involves depression of alveolar ventilation to accumulate CO2. However, the higher PaCO2 stimulates ventilation, which prevents complete compensation (return of pH to 7.4).

4. E. Renal compensation for acidosis, of both respiratory and metabolic origin (assuming the metabolic acidosis is not due to renal pathology), involves synthesis of NH3 and subsequent excretion of NH4+.

5. D. Renal compensation for acidosis involves bicarbonate reabsorption, not bicarbonate excretion.

6. D. After an hour, we should expect respiratory compensation but not renal compensation. Thus, there will be a low bicarbonate due mainly to the acid injection and low PaCO2 due to the hyperventilation.

7. C. The low pH indicates acidosis. The low bicarbonate indicates metabolic acidosis. The high PaCO2 indicates respiratory acidosis. Presumably, the narcotic overdose depressed the patient's respiratory center in the brain, reducing ventilation. The reduction of available oxygen then caused cellular anaerobic metabolism to increase, leading to production of lactic acid, etc.