Respiratory system: perfusion versus diffusion limitation: 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: perfusion versus diffusion limitation

Sample item

At rest, nitrous oxide equilibrates with capillary blood within the first part of its transit through the alveolar capillary, whereas carbon monoxide does not equilibrate. Which statement is correct?

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

The answer and the reasoning

D. Nitrous oxide uptake is perfusion-limited and carbon monoxide uptake is diffusion-limited

A gas that does not combine with haemoglobin, such as nitrous oxide, quickly reaches equilibrium, so its uptake depends on blood flow. Carbon monoxide binds haemoglobin so tightly that its partial pressure in plasma stays low and no equilibrium is reached, so its uptake depends on membrane diffusion. This is why carbon monoxide is used to measure diffusing capacity.

Option by option

  • A. Both gases are diffusion-limited

    Nitrous oxide equilibrates early, so its uptake cannot be diffusion-limited.

  • B. Both gases are perfusion-limited

    Carbon monoxide does not equilibrate, so its uptake is not limited by flow.

  • C. Nitrous oxide uptake is diffusion-limited and carbon monoxide uptake is perfusion-limited

    This reverses the correct assignment of the two gases.

  • D. Nitrous oxide uptake is perfusion-limited and carbon monoxide uptake is diffusion-limited (correct)

    Correct. Rapid equilibrium means perfusion limitation; avid haemoglobin binding means diffusion limitation.

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 D, Nitrous oxide uptake is perfusion-limited and carbon monoxide uptake is diffusion-limited. A gas that does not combine with haemoglobin, such as nitrous oxide, quickly reaches equilibrium, so its uptake depends on blood flow.

Why is "Both gases are diffusion-limited" not the answer?

Nitrous oxide equilibrates early, so its uptake cannot be diffusion-limited.

Why is "Both gases are perfusion-limited" not the answer?

Carbon monoxide does not equilibrate, so its uptake is not limited by flow.

Why is "Nitrous oxide uptake is diffusion-limited and carbon monoxide uptake…" not the answer?

This reverses the correct assignment of the two gases.

What concept does this question test?

It tests Respiratory system: perfusion versus diffusion limitation in Physiology. Carbon monoxide binds haemoglobin so tightly that its partial pressure in plasma stays low and no equilibrium is reached, so its uptake depends on membrane diffusion. This is why carbon monoxide is used to measure diffusing capacity.

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 Respiratory system: perfusion versus diffusion limitation?

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.