Respiratory system: physiological shunt in health: 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: physiological shunt in health

Sample item

In a healthy person breathing air, arterial PO2 is a few mmHg lower than the calculated ideal alveolar PO2 even with well-matched ventilation and perfusion. What is the main contribution?

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

The answer and the reasoning

C. Venous blood from the bronchial and thebesian veins draining into the left heart

A small proportion of cardiac output, mostly bronchial venous and some coronary (thebesian) blood, reaches the left side without passing through ventilated alveoli. This anatomical shunt mixes desaturated blood with oxygenated blood and produces a small normal alveolar-arterial gradient.

Option by option

  • A. Absorption of oxygen by alveolar macrophages

    Macrophage consumption of oxygen is negligible.

  • B. Complete diffusion limitation at rest

    At rest, diffusion equilibrium is reached long before the end of the capillary, so there is no diffusion limit.

  • C. Venous blood from the bronchial and thebesian veins draining into the left heart (correct)

    Correct. Bronchial and thebesian venous drainage bypasses the alveoli and lowers arterial PO2.

  • D. Failure of oxygen to dissolve in plasma

    Oxygen dissolves normally in plasma and is not the cause of the gradient.

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 C, Venous blood from the bronchial and thebesian veins draining into the left heart. A small proportion of cardiac output, mostly bronchial venous and some coronary (thebesian) blood, reaches the left side without passing through ventilated alveoli.

Why is "Absorption of oxygen by alveolar macrophages" not the answer?

Macrophage consumption of oxygen is negligible.

Why is "Complete diffusion limitation at rest" not the answer?

At rest, diffusion equilibrium is reached long before the end of the capillary, so there is no diffusion limit.

Why is "Failure of oxygen to dissolve in plasma" not the answer?

Oxygen dissolves normally in plasma and is not the cause of the gradient.

What concept does this question test?

It tests Respiratory system: physiological shunt in health in Physiology. This anatomical shunt mixes desaturated blood with oxygenated blood and produces a small normal alveolar-arterial gradient.

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: physiological shunt in health?

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.