Cardiovascular: haemodynamics: 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.

Try it

Physiology · Cardiovascular: haemodynamics

Sample item

A vasodilator drug doubles the radius of a systemic arteriole while the pressure gradient across it and the blood viscosity stay constant. By what factor does flow through that arteriole change?

AIt increases 8-foldBIt increases 2-foldCIt increases 4-foldDIt increases 16-fold

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

The answer and the reasoning

D. It increases 16-fold

According to Poiseuille's law, flow is proportional to the fourth power of the vessel radius, and resistance is inversely proportional to r to the power 4. Doubling the radius therefore gives 2 x 2 x 2 x 2 = 16 times the flow. This is why small changes in arteriolar calibre have such a large effect on peripheral resistance.

Option by option

A. It increases 8-fold
An 8-fold change would follow a cubic relationship, which does not describe laminar flow in a tube.
B. It increases 2-fold
A 2-fold change would occur only if flow were directly proportional to radius.
C. It increases 4-fold
A 4-fold change would occur if flow depended on the square of the radius, which is the cross-sectional area.
D. It increases 16-fold (correct)
Correct. Flow varies with r4, so 2 to the power 4 gives 16.

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 D, It increases 16-fold. According to Poiseuille's law, flow is proportional to the fourth power of the vessel radius, and resistance is inversely proportional to r to the power 4.

Why is "It increases 8-fold" not the answer?

An 8-fold change would follow a cubic relationship, which does not describe laminar flow in a tube.

Why is "It increases 2-fold" not the answer?

A 2-fold change would occur only if flow were directly proportional to radius.

Why is "It increases 4-fold" not the answer?

A 4-fold change would occur if flow depended on the square of the radius, which is the cross-sectional area.

What concept does this question test?

It tests Cardiovascular: haemodynamics in Physiology. Doubling the radius therefore gives 2 x 2 x 2 x 2 = 16 times the flow. This is why small changes in arteriolar calibre have such a large effect on peripheral resistance.

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: haemodynamics?

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.