Respiratory system: surface tension and lung recoil: 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: surface tension and lung recoil

Sample item

An excised lung is inflated and deflated first with air and then with saline. Compared with the air-filled lung, the saline-filled lung is much more compliant and shows almost no hysteresis. What does this show?

ASaline raises pleural pressure and stiffens the lungBSaline is incompressible, so the lung volume cannot changeCFilling with saline abolishes the air-liquid interface, leaving only tissue elastic recoilDSaline inactivates elastin fibres in the alveolar walls

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

The answer and the reasoning

C. Filling with saline abolishes the air-liquid interface, leaving only tissue elastic recoil

In air-filled lungs surface tension at the alveolar liquid layer contributes about two thirds of the elastic recoil and causes hysteresis. Filling with saline removes the interface, so recoil is due to tissue elastic forces alone, compliance rises and hysteresis nearly disappears.

Option by option

A. Saline raises pleural pressure and stiffens the lung
An excised lung has no pleural pressure; the change arises from the interface.
B. Saline is incompressible, so the lung volume cannot change
Saline-filled lungs can still change volume as liquid is added or removed, so incompressibility is not relevant.
C. Filling with saline abolishes the air-liquid interface, leaving only tissue elastic recoil (correct)
Correct. Removing the interface isolates tissue elastic recoil.
D. Saline inactivates elastin fibres in the alveolar walls
Saline does not damage elastin on this timescale.

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.

Questions about this Physiology question

What is the answer to this Physiology question?

The best answer is C, Filling with saline abolishes the air-liquid interface, leaving only tissue elastic recoil. In air-filled lungs surface tension at the alveolar liquid layer contributes about two thirds of the elastic recoil and causes hysteresis.

Why is "Saline raises pleural pressure and stiffens the lung" not the answer?

An excised lung has no pleural pressure; the change arises from the interface.

Why is "Saline is incompressible, so the lung volume cannot change" not the answer?

Saline-filled lungs can still change volume as liquid is added or removed, so incompressibility is not relevant.

Why is "Saline inactivates elastin fibres in the alveolar walls" not the answer?

Saline does not damage elastin on this timescale.

What concept does this question test?

It tests Respiratory system: surface tension and lung recoil in Physiology. Filling with saline removes the interface, so recoil is due to tissue elastic forces alone, compliance rises and hysteresis nearly disappears.

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: surface tension and lung recoil?

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.