Renal and acid-base: Henderson-Hasselbalch ratio: practice question

A Physiology question in the style of the NEET PG paper. Answer it first, then read why the right option is right and why each of the others is wrong.

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Physiology · Renal and acid-base: Henderson-Hasselbalch ratio

Sample item

In a healthy person, plasma pH is 7.4, bicarbonate is 24 mmol/L and PCO2 is 40 mmHg. Given that dissolved CO2 is 0.03 x PCO2, what is the ratio of bicarbonate to dissolved CO2?

Original practice item, not a recalled exam question. Nothing here is recorded.

The answer and the reasoning

A. 20 to 1

Dissolved CO2 is 0.03 x 40, which is 1.2 mmol/L. The ratio of 24 to 1.2 is 20 to 1, and the Henderson-Hasselbalch equation gives pH = 6.1 + log 20, which is 6.1 + 1.3 = 7.4. A change in this ratio, not in either value alone, determines the pH.

Option by option

  • A. 20 to 1 (correct)

    Correct. 24 divided by 1.2 is 20, and log 20 is about 1.3.

  • B. 10 to 1

    A ratio of 10 to 1 would give a pH of 7.1.

  • C. 1 to 20

    A ratio of 1 to 20 would give a very acidic pH.

  • D. 40 to 1

    A ratio of 40 to 1 would give a pH of about 7.7.

Source note: Original NextPrep sample item — pending expert review. This item is original practice material awaiting independent expert review, so confirm important facts in a standard textbook.

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Questions about this Physiology question

What is the answer to this Physiology question?

The best answer is A, 20 to 1. Dissolved CO2 is 0.

Why is "10 to 1" not the answer?

A ratio of 10 to 1 would give a pH of 7.1.

Why is "1 to 20" not the answer?

A ratio of 1 to 20 would give a very acidic pH.

Why is "40 to 1" not the answer?

A ratio of 40 to 1 would give a pH of about 7.7.

What concept does this question test?

It tests Renal and acid-base: Henderson-Hasselbalch ratio in Physiology. 03 x 40, which is 1.2 mmol/L. The ratio of 24 to 1.2 is 20 to 1, and the Henderson-Hasselbalch equation gives pH = 6.1 + log 20, which is 6.1 + 1.3 = 7.4. A change in this ratio, not in either value alone, determines the pH.

How hard is this question?

We tag it as an application question: it asks you to use a fact on a patient or a finding. The tag is our own editorial judgement, not an official difficulty rating, so read the reason for every option, not only the answer.

How should I study Renal and acid-base: Henderson-Hasselbalch ratio?

For each system, sketch the key graph (pressure–volume loop, oxygen dissociation curve, nephron handling) and explain it aloud. Then use the flashcards and the other Physiology questions on this site to test what stuck.

Is this a real NEET PG question?

No. It is an original practice item (editorial status: sample, version 1). NextPrep does not publish recalled or leaked questions, and independent faculty review of this item has not been completed yet, so confirm important facts in a standard textbook.