Respiratory system: diffusion of carbon dioxide: 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: diffusion of carbon dioxide

Sample item

Carbon dioxide diffuses across the alveolar-capillary membrane about 20 times faster than oxygen, although its partial pressure gradient is much smaller. What is the main reason?

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

The answer and the reasoning

B. Its much higher solubility in the membrane and tissue fluid

By Graham's law, diffusion rate is proportional to solubility and inversely proportional to the square root of molecular weight. CO2 is slightly heavier than O2, but about 20 times more soluble, so its diffusion coefficient is about 20 times greater. This is why CO2 retention is rarely due to diffusion impairment.

Option by option

  • A. Facilitated transport by carbonic anhydrase in the membrane

    Carbonic anhydrase acts in red cells for transport and is not responsible for the diffusion across the membrane.

  • B. Its much higher solubility in the membrane and tissue fluid (correct)

    Correct. High solubility accounts for the faster diffusion of CO2.

  • C. Its lower molecular weight

    CO2 has a higher molecular weight (44) than O2 (32).

  • D. Its larger partial pressure gradient

    The CO2 gradient is only a few mmHg, much less than the gradient for oxygen.

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, Its much higher solubility in the membrane and tissue fluid. By Graham's law, diffusion rate is proportional to solubility and inversely proportional to the square root of molecular weight.

Why is "Facilitated transport by carbonic anhydrase in the membrane" not the answer?

Carbonic anhydrase acts in red cells for transport and is not responsible for the diffusion across the membrane.

Why is "Its lower molecular weight" not the answer?

CO2 has a higher molecular weight (44) than O2 (32).

Why is "Its larger partial pressure gradient" not the answer?

The CO2 gradient is only a few mmHg, much less than the gradient for oxygen.

What concept does this question test?

It tests Respiratory system: diffusion of carbon dioxide in Physiology. CO2 is slightly heavier than O2, but about 20 times more soluble, so its diffusion coefficient is about 20 times greater. This is why CO2 retention is rarely due to diffusion impairment.

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: diffusion of carbon dioxide?

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.