Physiology, MBBS Second Year. This chapter is split into 10 topics and has
516 practice questions. The topics covered are listed below, followed by
10 free sample questions from the chapter.
Topics in this chapter
Acid-base fundamentals52 questions
Chemical buffer systems50 questions
New HCO3 generation: phosphate & ammonia buffers50 questions
Acid-base and electrolyte interactions50 questions
Sample questions: Acid-Base Regulation
10 sample questions with the correct answer and an explanation. Try to answer each one before reading the explanation.
Q1. Which buffer is responsible for most of the buffering of acid in urine?
A. Urate buffer
B. Citrate buffer
C. Phosphate buffer
D. Ammonia buffer(correct answer)
Answer: D.The ammonia buffer accounts for most urinary buffering (about 50% of acid excreted and 50% of new bicarbonate generated); phosphate is a strong buffer but much of it is reabsorbed, while urate and citrate are weak buffers.
Q2. Generation of 'new' bicarbonate occurs when secreted H+ in the tubular fluid:
A. Combines with buffers OTHER than bicarbonate(correct answer)
B. Is reabsorbed into the blood
C. Combines with filtered bicarbonate
D. Combines with water
Answer: A.H+ combining with filtered bicarbonate merely reclaims existing bicarbonate; when H+ instead combines with non-bicarbonate buffers (phosphate, ammonia), a NEW bicarbonate is added to the blood — the net gain that replaces buffered acid losses.
Q3. In CHRONIC acidosis, the dominant mechanism for acid elimination becomes:
A. Excretion of bicarbonate
B. Excretion of titratable acid only
C. Reduced H+ secretion
D. Excretion of ammonium (NH4+)(correct answer)
Answer: D.Chronic acidosis markedly increases glutamine metabolism and ammonium excretion, so NH4+ excretion becomes the dominant route of acid elimination — this is why urinary ammonium rises sharply in chronic metabolic acidosis.
Q4. Which of the following causes metabolic acidosis through EXCESS formation of metabolic acids?
A. Diabetes mellitus (ketoacidosis)(correct answer)
B. Hyperaldosteronism
C. Diarrhea
D. Gastric vomiting
Answer: A.Diabetes mellitus (and starvation) generates excess metabolic acids such as acetoacetic acid from fat metabolism; diarrhea and renal tubular disorders cause bicarbonate loss, and vomiting causes acid loss (alkalosis).
Q5. A patient with metabolic alkalosis from prolonged vomiting has an acidic urine. Why?
A. Urinary pH is unrelated to body pH
B. Bicarbonate is being excreted
C. The kidney cannot acidify urine
D. Volume depletion and aldosterone increase H+ secretion, so the kidney excretes acid despite systemic alkalosis(correct answer)
Answer: D.Vomiting causes volume and chloride depletion, activating aldosterone and increasing distal H+ secretion; the kidney paradoxically excretes acid (and reabsorbs bicarbonate), perpetuating the alkalosis — 'paradoxical aciduria'.
Q6. Assertion: Measuring the anion gap helps identify the cause of a metabolic acidosis. Reason: Acidoses caused by unmeasured anion accumulation raise the gap, whereas bicarbonate loss does not.
A. Assertion is true but Reason is false
B. Assertion is false but Reason is true
C. Both are true, but the Reason does not explain the Assertion
D. Both Assertion and Reason are true, and the Reason correctly explains the Assertion(correct answer)
Answer: D.Because the gap reflects unmeasured anions, acidoses that accumulate them (ketoacids, lactate, toxins) widen it, while bicarbonate loss with chloride retention leaves it normal — making the gap a key diagnostic discriminator.
Q7. A patient with pneumonia develops tachypnea and a low PCO2. Which is the most likely explanation?
A. Renal failure
B. Respiratory compensation with hyperventilation(correct answer)
C. CO2 retention
D. Metabolic acidosis
Answer: B.Pneumonia can impair gas exchange, and the resulting hypoxemia and/or acidosis stimulate hyperventilation, lowering PCO2 — respiratory compensation in action (though CO2 retention may later occur if ventilation fails).
Q8. Which statement correctly describes tubular fluid bicarbonate handling?
A. Filtered bicarbonate disappears from tubular fluid as its H+ is re-added and CO2 reforms(correct answer)
B. Bicarbonate is secreted to correct acidosis
C. Bicarbonate is not filtered
D. Bicarbonate is transported intact into the cell
Answer: A.In the lumen, secreted H+ combines with filtered HCO3− to form H2CO3, which carbonic anhydrase converts to CO2 and water; CO2 enters the cell and bicarbonate is regenerated and returned to blood — so bicarbonate appears to be reabsorbed.
Q9. Assertion: The kidneys are the only organs that can replenish bicarbonate consumed in buffering. Reason: H+ excreted with urinary buffers generates new bicarbonate entering the blood.
A. Assertion is false but Reason is true
B. Both Assertion and Reason are true, and the Reason correctly explains the Assertion(correct answer)
C. Both are true, but the Reason does not explain the Assertion
D. Assertion is true but Reason is false
Answer: B.When secreted H+ is excreted bound to buffers (phosphate, ammonia) rather than to filtered bicarbonate, a new HCO3− is added to blood — replacing the bicarbonate consumed when fixed acids were buffered. Only the kidney can do this.
Q10. Which sequence describes bicarbonate reabsorption in the proximal tubule?
A. H+ enters cell → HCO3− secreted
B. HCO3− enters cell intact → H+ reabsorbed
C. CO2 is secreted into the lumen
D. CO2 enters cell → H2CO3 forms → H+ secreted into lumen → combines with filtered HCO3− → HCO3− regenerated and returned to blood(correct answer)
Answer: D.CO2 entering the cell forms H2CO3 (via carbonic anhydrase); H+ is secreted to combine with luminal bicarbonate, and a new HCO3− formed inside the cell passes into the blood — the net effect being bicarbonate reabsorption.