Respiratory system: helium dilution: 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 · Respiratory system: helium dilution

Sample item

In a closed-circuit helium dilution test, a 4 L spirometer contains 8% helium. After the patient rebreathes from it at the end of a normal expiration, the helium concentration equilibrates at 5%. What is the patient's functional residual capacity?

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

The answer and the reasoning

B. 2.4 L

The amount of helium is conserved as it mixes into the lungs, so C1 x V1 = C2 x (V1 + FRC). Thus 0.08 x 4 = 0.05 x (4 + FRC), which gives 4 + FRC = 6.4 and FRC = 2.4 L. The method measures only gas communicating with the airways, so it underestimates FRC with air trapping.

Option by option

  • A. 4.0 L

    4.0 L is the spirometer volume, which does not include the lung gas.

  • B. 2.4 L (correct)

    0.32 = 0.05 x (4 + FRC), so FRC = 6.4 - 4 = 2.4 L.

  • C. 1.6 L

    1.6 L is simply the ratio 8/5 and ignores the spirometer volume.

  • D. 6.4 L

    6.4 L is the final combined volume; the spirometer volume must still be subtracted.

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 B, 2.4 L. The amount of helium is conserved as it mixes into the lungs, so C1 x V1 = C2 x (V1 + FRC).

Why is "4.0 L" not the answer?

4.0 L is the spirometer volume, which does not include the lung gas.

Why is "1.6 L" not the answer?

1.6 L is simply the ratio 8/5 and ignores the spirometer volume.

Why is "6.4 L" not the answer?

6.4 L is the final combined volume; the spirometer volume must still be subtracted.

What concept does this question test?

It tests Respiratory system: helium dilution in Physiology. Thus 0.08 x 4 = 0.05 x (4 + FRC), which gives 4 + FRC = 6.4 and FRC = 2.4 L. The method measures only gas communicating with the airways, so it underestimates FRC with air trapping.

How hard is this question?

We tag it as a multi-step question: it asks you to combine more than one idea. 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 Respiratory system: helium dilution?

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.