Respiratory system: plethysmography versus 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: plethysmography versus helium dilution

Sample item

In a smoker with large emphysematous bullae, functional residual capacity measured by body plethysmography is much larger than that measured by helium dilution. What explains the difference?

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

The answer and the reasoning

B. Plethysmography includes trapped gas that does not communicate with the airways

Plethysmography applies Boyle's law to the whole thoracic gas volume, including poorly ventilated or trapped air in bullae. Helium dilution reaches only gas that communicates with the airways within the test time, so it underestimates volume in obstructive disease. The gap between the two methods estimates trapped gas.

Option by option

  • A. Helium rapidly equilibrates with all thoracic gas in emphysema

    In emphysema equilibration is slow and incomplete, which is why helium underestimates.

  • B. Plethysmography includes trapped gas that does not communicate with the airways (correct)

    Correct. Boyle's law captures all thoracic gas, including bullae, whereas helium reaches only communicating spaces.

  • C. Helium dissolves in blood and is lost from the lung

    Helium is poorly soluble in blood, so it is not significantly lost.

  • D. Plethysmography measures only the anatomical dead space

    Plethysmography measures thoracic gas volume, not dead space.

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, Plethysmography includes trapped gas that does not communicate with the airways. Plethysmography applies Boyle's law to the whole thoracic gas volume, including poorly ventilated or trapped air in bullae.

Why is "Helium rapidly equilibrates with all thoracic gas in emphysema" not the answer?

In emphysema equilibration is slow and incomplete, which is why helium underestimates.

Why is "Helium dissolves in blood and is lost from the lung" not the answer?

Helium is poorly soluble in blood, so it is not significantly lost.

Why is "Plethysmography measures only the anatomical dead space" not the answer?

Plethysmography measures thoracic gas volume, not dead space.

What concept does this question test?

It tests Respiratory system: plethysmography versus helium dilution in Physiology. Helium dilution reaches only gas that communicates with the airways within the test time, so it underestimates volume in obstructive disease. The gap between the two methods estimates trapped gas.

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: plethysmography versus 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.