Respiratory system: closing volume and age: 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: closing volume and age

Sample item

A 72-year-old woman is mildly hypoxaemic when lying supine under sedation, although her chest radiograph is clear. Which mechanism best explains this?

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

The answer and the reasoning

A. Closing volume has risen above the supine FRC, so small airways close during tidal breathing

With age, loss of elastic recoil allows dependent small airways to close at higher lung volumes, so closing volume rises. When it exceeds the FRC, as it can in the supine position, those units are perfused but not ventilated during tidal breathing, producing low V/Q regions and hypoxaemia.

Option by option

  • A. Closing volume has risen above the supine FRC, so small airways close during tidal breathing (correct)

    Correct. Airway closure at a volume above FRC leaves perfused alveoli unventilated.

  • B. Her vital capacity has risen with age and alters gas exchange

    Vital capacity falls, rather than rises, with age.

  • C. Surfactant production has increased and stiffens the alveoli

    Surfactant excess is not a feature of ageing and does not stiffen the lungs.

  • D. Hypoxic pulmonary vasoconstriction has failed globally

    Hypoxic vasoconstriction would help match perfusion, and its global failure does not account for the age and posture effect.

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, Closing volume has risen above the supine FRC, so small airways close during tidal breathing. With age, loss of elastic recoil allows dependent small airways to close at higher lung volumes, so closing volume rises.

Why is "Her vital capacity has risen with age and alters gas exchange" not the answer?

Vital capacity falls, rather than rises, with age.

Why is "Surfactant production has increased and stiffens the alveoli" not the answer?

Surfactant excess is not a feature of ageing and does not stiffen the lungs.

Why is "Hypoxic pulmonary vasoconstriction has failed globally" not the answer?

Hypoxic vasoconstriction would help match perfusion, and its global failure does not account for the age and posture effect.

What concept does this question test?

It tests Respiratory system: closing volume and age in Physiology. When it exceeds the FRC, as it can in the supine position, those units are perfused but not ventilated during tidal breathing, producing low V/Q regions and hypoxaemia.

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: closing volume and age?

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.