Respiratory system: Bohr dead space: 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: Bohr dead space

Sample item

In a resting adult, arterial PCO2 is 40 mmHg and mixed expired PCO2 is 28 mmHg. Tidal volume is 500 mL. Using the Bohr equation, what is the physiological dead space?

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

The answer and the reasoning

B. 150 mL

The Bohr equation is VD/VT = (PaCO2 - PECO2)/PaCO2. Here it is (40 - 28)/40 = 12/40 = 0.30. Multiplied by a tidal volume of 500 mL, the physiological dead space is 150 mL. It is the sum of anatomical dead space and alveolar dead space from ventilated but unperfused alveoli.

Option by option

  • A. 120 mL

    120 mL would require a ratio of 0.24, which does not follow from these values.

  • B. 150 mL (correct)

    0.30 x 500 mL = 150 mL.

  • C. 250 mL

    250 mL would correspond to a ratio of 0.5, requiring a mixed expired PCO2 of 20 mmHg.

  • D. 350 mL

    350 mL is the fraction that reaches perfused alveoli (0.70 x 500), the complement of dead space.

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, 150 mL. The Bohr equation is VD/VT = (PaCO2 - PECO2)/PaCO2.

Why is "120 mL" not the answer?

120 mL would require a ratio of 0.24, which does not follow from these values.

Why is "250 mL" not the answer?

250 mL would correspond to a ratio of 0.5, requiring a mixed expired PCO2 of 20 mmHg.

Why is "350 mL" not the answer?

350 mL is the fraction that reaches perfused alveoli (0.70 x 500), the complement of dead space.

What concept does this question test?

It tests Respiratory system: Bohr dead space in Physiology. Here it is (40 - 28)/40 = 12/40 = 0.30. Multiplied by a tidal volume of 500 mL, the physiological dead space is 150 mL. It is the sum of anatomical dead space and alveolar dead space from ventilated but unperfused alveoli.

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: Bohr dead space?

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.