Questions: Acid-Base Balance and Three Regulatory Systems

Question 1

A patient with diabetic ketoacidosis has pH 7.20, HCO3− 10 mEq/L (normal 24), and PCO2 30 mmHg (normal 40). Which regulatory system is providing the most immediate large-scale compensation here, and what is it doing?

• A. The kidneys are excreting acid and retaining bicarbonate to restore the HCO3− deficit
• B. Chemical buffers have restored pH to near-normal by absorbing the excess ketoacids
• C. The respiratory system has increased ventilation to reduce PCO2, partially compensating for the lost bicarbonate
• D. The chemical buffer and respiratory systems together have fully corrected the pH disturbance

Answer: (C)

This is respiratory compensation for metabolic acidosis. The low PCO2 (30, below normal 40) indicates hyperventilation is in progress — the respiratory system is blowing off CO2 to shift the Henderson-Hasselbalch ratio back toward normal. Respiratory compensation operates over minutes and is the fastest significant regulatory response after the initial buffer action. The kidneys (option A) would provide more complete compensation but require hours to days. Buffers (option B) absorbed some initial acid but did not restore pH — it's still 7.20. Option D is wrong; compensation here is partial, not complete.

Question 2

A patient hyperventilates due to anxiety for 30 minutes (PCO2 falls from 40 to 25 mmHg). Before any renal compensation can occur, what happens to their blood pH according to the Henderson-Hasselbalch equation?

• A. pH falls, because hyperventilation depletes bicarbonate
• B. pH rises, because reducing PCO2 shifts the HCO3−/CO2 ratio upward
• C. pH stays the same, because chemical buffers immediately counteract the CO2 loss
• D. pH falls, because CO2 is an acid and removing it makes the blood less acidic

Answer: (B)

The Henderson-Hasselbalch equation: pH = 6.1 + log([HCO3−] / 0.03 × PCO2). Reducing PCO2 (the denominator of the ratio) increases the ratio, increasing the log term, and raising pH — this is respiratory alkalosis. Chemical buffers (option C) provide some resistance but cannot fully counteract the shift. Option D contains a logical error: CO2 is indeed acidic (forms H2CO3), so *removing* it raises pH, not lowers it. Option A is wrong — hyperventilation doesn't deplete bicarbonate quickly; the initial change is a CO2 shift.

Question 3

Chemical buffers in the blood solve the acid-base problem by permanently neutralizing excess acid, restoring pH to normal without requiring any action from the respiratory or renal systems.

• T. True
• F. False

Answer: False

Buffers resist pH change but do not restore it. When a buffer pair (e.g., HCO3−/H2CO3) absorbs an acid load, it is consumed in the process — converting the strong acid to the weak acid form. pH improves relative to what it would have been, but it is not restored to normal, and the buffer capacity is partially depleted. Buffers buy time for the respiratory and renal systems to respond. Full compensation requires either the lungs to adjust PCO2 or the kidneys to regenerate bicarbonate. Saying buffers 'solve the problem' is like saying shock absorbers repair a pothole.

Question 4

The respiratory system can compensate for metabolic acidosis (low HCO3−) by hyperventilating to reduce PCO2, but cannot fully restore normal acid-base balance because it cannot regenerate the bicarbonate that was consumed.

• T. True
• F. False

Answer: True

This is a fundamental constraint of respiratory compensation. The Henderson-Hasselbalch equation shows pH depends on the HCO3−/PCO2 ratio. Respiratory compensation adjusts the PCO2 side of the ratio — powerfully and quickly — but it cannot increase HCO3−. In metabolic acidosis, the lost bicarbonate can only be replaced by the kidneys, which generate new HCO3− by excreting H+ bound to urinary buffers (phosphate, ammonia). This is why complete correction of metabolic acidosis requires renal compensation over hours to days, even after respiratory compensation has partially normalized the ratio.

Question 5

Explain the complementary roles of the chemical buffer system, the respiratory system, and the renal system by describing what each one specifically changes in the Henderson-Hasselbalch equation, and on what timescale.

Model answer: The Henderson-Hasselbalch equation is: pH = 6.1 + log([HCO3−] / 0.03 × PCO2). Chemical buffers (seconds) convert strong acids to weak acids, reducing the size of the pH shift without specifically altering either HCO3− or PCO2 — they absorb the H+ before it affects the ratio fully. The respiratory system (minutes) controls PCO2 by changing ventilation rate: hyperventilation lowers PCO2 (raises the ratio, raises pH); hypoventilation raises PCO2 (lowers ratio, lowers pH). The kidneys (hours to days) control HCO3−: they can reabsorb more bicarbonate, generate new bicarbonate by excreting H+, or excrete excess bicarbonate. Only the kidneys can restore a depleted bicarbonate pool — and only renal compensation can fully correct a metabolic disturbance.

Understanding that the three systems manipulate different variables in the same equation — and on different timescales — is the core clinical skill. Acid-base analysis requires asking: which system created the primary disturbance (high/low PCO2 or high/low HCO3−?), and is the compensating system responding as expected (using formulas like Winter's to check)?