Respiratory: surfactant and alveolar stability: 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: surfactant and alveolar stability

Sample item

Without surfactant, two connected alveoli of different radii have the same surface tension. According to the Laplace relation, what happens, and how does surfactant help?

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

The answer and the reasoning

A. Pressure is higher in the smaller alveolus, so it empties into the larger one; surfactant lowers tension more in the smaller alveolus

Laplace law for a sphere gives pressure equal to 2T divided by r, so at equal tension a smaller alveolus has a higher pressure and tends to collapse into the larger one. Surfactant (dipalmitoylphosphatidylcholine) is concentrated as the alveolus shrinks, so it reduces tension more in the smaller alveolus and stabilises it. It also raises compliance and keeps alveoli dry.

Option by option

  • A. Pressure is higher in the smaller alveolus, so it empties into the larger one; surfactant lowers tension more in the smaller alveolus (correct)

    Correct. Inverse radius dependence plus density-dependent action stabilises small alveoli.

  • B. Pressure is higher in the smaller alveolus, so it empties into the larger one; surfactant increases tension in the larger alveolus

    Surfactant reduces tension and never raises it.

  • C. Pressure is equal in both, so flow does not occur; surfactant is needed only for compliance

    Pressures differ with radius, so collapse of small alveoli is the real problem.

  • D. Pressure is higher in the larger alveolus, so it empties into the smaller one; surfactant lowers tension equally in both

    Pressure is inversely proportional to radius, so the smaller alveolus has the higher pressure.

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, Pressure is higher in the smaller alveolus, so it empties into the larger one; surfactant lowers tension more in the smaller alveolus. Laplace law for a sphere gives pressure equal to 2T divided by r, so at equal tension a smaller alveolus has a higher pressure and tends to collapse into the larger one.

Why is "Pressure is higher in the smaller alveolus, so it empties into the la…" not the answer?

Surfactant reduces tension and never raises it.

Why is "Pressure is equal in both, so flow does not occur; surfactant is need…" not the answer?

Pressures differ with radius, so collapse of small alveoli is the real problem.

Why is "Pressure is higher in the larger alveolus, so it empties into the sma…" not the answer?

Pressure is inversely proportional to radius, so the smaller alveolus has the higher pressure.

What concept does this question test?

It tests Respiratory: surfactant and alveolar stability in Physiology. Surfactant (dipalmitoylphosphatidylcholine) is concentrated as the alveolus shrinks, so it reduces tension more in the smaller alveolus and stabilises it. It also raises compliance and keeps alveoli dry.

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: surfactant and alveolar stability?

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.