Renal: Starling forces in glomerular filtration: 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 · Renal: Starling forces in glomerular filtration

Sample item

In a patient, glomerular capillary hydrostatic pressure is 60 mmHg, plasma oncotic pressure in the capillary is 32 mmHg and Bowman space pressure is 18 mmHg. A ureteric stone then raises Bowman space pressure to 28 mmHg with other forces unchanged. What happens to the net filtration pressure?

AIt falls from 10 mmHg to about 0 mmHg, so filtration nearly ceasesBIt falls from 10 mmHg to minus 10 mmHg, so fluid is absorbed from Bowman spaceCIt remains 10 mmHg, because hydrostatic pressure in the capillary is unchangedDIt rises from 10 mmHg to 20 mmHg

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

The answer and the reasoning

A. It falls from 10 mmHg to about 0 mmHg, so filtration nearly ceases

Net filtration pressure equals glomerular hydrostatic pressure minus the sum of plasma oncotic pressure and Bowman space pressure. At first it is 60 minus (32 + 18), which gives 10 mmHg, and after obstruction it is 60 minus (32 + 28), which gives 0 mmHg. Thus GFR falls towards zero in acute obstruction.

Option by option

A. It falls from 10 mmHg to about 0 mmHg, so filtration nearly ceases (correct)
Correct. A 10 mmHg rise in back pressure abolishes the net driving force.
B. It falls from 10 mmHg to minus 10 mmHg, so fluid is absorbed from Bowman space
The calculation gives 0 mmHg, not minus 10 mmHg.
C. It remains 10 mmHg, because hydrostatic pressure in the capillary is unchanged
Bowman space pressure is one of the Starling forces and changes the net pressure.
D. It rises from 10 mmHg to 20 mmHg
Bowman space pressure opposes filtration, so raising it cannot increase the net pressure.

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, It falls from 10 mmHg to about 0 mmHg, so filtration nearly ceases. Net filtration pressure equals glomerular hydrostatic pressure minus the sum of plasma oncotic pressure and Bowman space pressure.

Why is "It falls from 10 mmHg to minus 10 mmHg, so fluid is absorbed from Bow…" not the answer?

The calculation gives 0 mmHg, not minus 10 mmHg.

Why is "It remains 10 mmHg, because hydrostatic pressure in the capillary is…" not the answer?

Bowman space pressure is one of the Starling forces and changes the net pressure.

Why is "It rises from 10 mmHg to 20 mmHg" not the answer?

Bowman space pressure opposes filtration, so raising it cannot increase the net pressure.

What concept does this question test?

It tests Renal: Starling forces in glomerular filtration in Physiology. At first it is 60 minus (32 + 18), which gives 10 mmHg, and after obstruction it is 60 minus (32 + 28), which gives 0 mmHg. Thus GFR falls towards zero in acute obstruction.

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 Renal: Starling forces in glomerular filtration?

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.