Cardiovascular: turbulent flow and anaemia: 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 · Cardiovascular: turbulent flow and anaemia

Sample item

A 25-year-old woman with a haemoglobin of 5 g/dL has a soft systolic murmur over the precordium and no structural heart disease. Which change in the Reynolds number explains the turbulence?

ALower blood viscosity and higher flow velocity both raise the Reynolds numberBNarrower vessel diameter with unchanged velocity lowers the Reynolds numberCLower flow velocity raises the Reynolds numberDHigher blood viscosity raises the Reynolds number

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

The answer and the reasoning

A. Lower blood viscosity and higher flow velocity both raise the Reynolds number

Reynolds number is the product of density, vessel diameter and velocity divided by viscosity, and flow becomes turbulent above about 2000 to 3000. Anaemia reduces viscosity and the hyperdynamic circulation raises velocity, so the number rises and a flow murmur is heard. The murmur resolves when the anaemia is corrected.

Option by option

A. Lower blood viscosity and higher flow velocity both raise the Reynolds number (correct)
Correct. Lower viscosity and faster flow both push the number above the critical value.
B. Narrower vessel diameter with unchanged velocity lowers the Reynolds number
Reynolds number is proportional to diameter, so a narrower vessel at the same velocity gives a lower number, which is not the situation in anaemia.
C. Lower flow velocity raises the Reynolds number
Slower flow would lessen turbulence, not cause it.
D. Higher blood viscosity raises the Reynolds number
Anaemia lowers viscosity, and higher viscosity would reduce the Reynolds number.

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 A, Lower blood viscosity and higher flow velocity both raise the Reynolds number. Reynolds number is the product of density, vessel diameter and velocity divided by viscosity, and flow becomes turbulent above about 2000 to 3000.

Why is "Narrower vessel diameter with unchanged velocity lowers the Reynolds…" not the answer?

Reynolds number is proportional to diameter, so a narrower vessel at the same velocity gives a lower number, which is not the situation in anaemia.

Why is "Lower flow velocity raises the Reynolds number" not the answer?

Slower flow would lessen turbulence, not cause it.

Why is "Higher blood viscosity raises the Reynolds number" not the answer?

Anaemia lowers viscosity, and higher viscosity would reduce the Reynolds number.

What concept does this question test?

It tests Cardiovascular: turbulent flow and anaemia in Physiology. Anaemia reduces viscosity and the hyperdynamic circulation raises velocity, so the number rises and a flow murmur is heard. The murmur resolves when the anaemia is corrected.

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 Cardiovascular: turbulent flow and anaemia?

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.