General physiology: body fluid compartment volumes: 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 · General physiology: body fluid compartment volumes

Sample item

In a healthy 70 kg man, total body water measured by deuterium oxide is 42 L, extracellular fluid space by inulin is 14 L, and plasma volume by Evans blue is 3 L. His haematocrit is 0.40. Ignoring trapped plasma, what are his intracellular fluid volume, interstitial fluid volume and blood volume?

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

The answer and the reasoning

A. 28 L, 11 L and 5 L

Intracellular fluid = total body water minus ECF = 42 - 14 = 28 L. Interstitial fluid = ECF minus plasma = 14 - 3 = 11 L. Blood volume = plasma volume / (1 - haematocrit) = 3 / 0.60 = 5 L.

Option by option

  • A. 28 L, 11 L and 5 L (correct)

    Correct. ICF 28 L, ISF 11 L and blood volume 5 L.

  • B. 28 L, 14 L and 5 L

    Interstitial fluid is ECF minus plasma (11 L), not the whole 14 L of ECF.

  • C. 14 L, 11 L and 5 L

    Intracellular volume is total body water minus ECF (28 L), not equal to the ECF.

  • D. 28 L, 11 L and 7.5 L

    Dividing plasma volume by the haematocrit (3/0.4) instead of by the plasma fraction (0.6) wrongly gives 7.5 L.

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, 28 L, 11 L and 5 L. Intracellular fluid = total body water minus ECF = 42 - 14 = 28 L.

Why is "28 L, 14 L and 5 L" not the answer?

Interstitial fluid is ECF minus plasma (11 L), not the whole 14 L of ECF.

Why is "14 L, 11 L and 5 L" not the answer?

Intracellular volume is total body water minus ECF (28 L), not equal to the ECF.

Why is "28 L, 11 L and 7.5 L" not the answer?

Dividing plasma volume by the haematocrit (3/0.4) instead of by the plasma fraction (0.6) wrongly gives 7.5 L.

What concept does this question test?

It tests General physiology: body fluid compartment volumes in Physiology. Interstitial fluid = ECF minus plasma = 14 - 3 = 11 L. Blood volume = plasma volume / (1 - haematocrit) = 3 / 0.60 = 5 L.

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 General physiology: body fluid compartment volumes?

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.