A patient with severe diabetic ketoacidosis has pH 7.2 and elevated plasma H⁺. Which compensatory change would you expect in their arterial blood gas?
AElevated pCO₂ — the lungs retain CO₂ to buffer excess acid
BReduced pCO₂ — hyperventilation blows off CO₂ to consume protons
CElevated HCO₃⁻ — the kidneys rapidly generate more bicarbonate within minutes
DNo change in pCO₂ — respiratory compensation only occurs in respiratory disorders
Metabolic acidosis triggers brainstem-mediated hyperventilation, which blows off CO₂. Because CO₂ + H₂O ↔ H₂CO₃ ↔ H⁺ + HCO₃⁻, removing CO₂ pulls the equilibrium left, consuming protons and raising pH. This respiratory compensation begins within minutes. Option A is wrong — retaining CO₂ would worsen acidosis. Option C is wrong because renal compensation takes hours to days, not minutes. Option D is wrong because respiratory compensation is the primary acute response to metabolic acid-base disorders.
Question 2 Multiple Choice
What makes the bicarbonate buffer system more effective than a typical closed chemical buffer in blood?
ABicarbonate has a higher pKa, making it more effective near physiological pH
BThe body can independently manipulate both ends of the equilibrium — CO₂ via breathing and HCO₃⁻ via the kidneys
CBicarbonate is present in higher concentrations than any other buffer in plasma
DIt is the only buffer system that directly neutralizes strong acids without producing any byproducts
The key advantage is that the bicarbonate system is open: CO₂ is a volatile gas regulated by ventilation rate, and HCO₃⁻ is regulated by renal reabsorption and secretion. Controlling both sides independently gives the body far greater buffering range than a closed system. Options A and C are factually incorrect (bicarbonate's pKa of 6.1 is actually suboptimal for 7.4, yet the system is powerful precisely because it is open). Option D is wrong — the neutralization products include CO₂ and water.
Question 3 True / False
Respiratory compensation for metabolic acidosis acts faster than renal compensation.
TTrue
FFalse
Answer: True
Respiratory compensation via altered ventilation rate begins within minutes of a pH change, as the brainstem's chemoreceptors rapidly detect rising CO₂ and falling pH. Renal compensation — increasing H⁺ secretion and bicarbonate reabsorption — requires hours to days to achieve maximal effect. This difference in timescale is clinically important: in acute metabolic acidosis, the respiratory system acts first as a bridge while the slower but more complete renal correction catches up.
Question 4 True / False
In metabolic acidosis, if respiratory compensation is working effectively, the patient's blood pH will return substantially to 7.4.
TTrue
FFalse
Answer: False
Respiratory compensation can only partially offset a metabolic acid-base disorder — it never fully corrects it and never overshoots. If the pH were fully corrected to 7.4, the brainstem stimulus for hyperventilation would disappear and breathing would normalize, allowing pH to fall again. The compensation stabilizes at a new lower pCO₂ and a pH between 7.4 and the nadir of the disorder, buying time for the kidneys to achieve more complete correction. Full normalization requires renal compensation.
Question 5 Short Answer
Why is it clinically significant that acid-base compensation never overshoots — i.e., why can't respiratory compensation cause alkalosis in a patient with metabolic acidosis?
Think about your answer, then reveal below.
Model answer: Compensation is driven by the deviation from normal pH. As hyperventilation reduces CO₂ and pH rises toward 7.4, the brainstem stimulus diminishes, slowing ventilation. The system reaches a new equilibrium short of full correction. If compensation fully corrected pH to 7.4, the stimulus would vanish and the system would revert. This self-limiting negative feedback prevents compensation from becoming an independent primary disorder.
This is critical for clinical interpretation of arterial blood gases. If you see a patient with metabolic acidosis and an alkaline pH, that cannot be explained by compensation — it indicates a second, independent primary disorder (respiratory alkalosis) occurring simultaneously. Recognizing this prevents misattributing two simultaneous primary disorders to compensation, which would lead to treating only one cause.