General physiology: effective osmoles and tonicity: practice question

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

The answer and the reasoning

C. Little net water shift occurs, because urea crosses cell membranes freely and is an ineffective osmole

Tonicity depends only on solutes that cannot freely cross the membrane. Urea equilibrates across most cell membranes, so it raises osmolality without creating a lasting transmembrane gradient. Effective osmoles such as sodium, glucose and mannitol are the ones that move water between compartments.

Option by option

• A. Cells shrink at first and stay shrunken, because urea is not metabolised

  Persistent shrinkage would need a persistent gradient, which free urea permeability prevents.

• B. Cells shrink, because the rise in measured osmolality creates an effective osmotic gradient

  A raised measured osmolality only shrinks cells if the solute is impermeant, as sodium or glucose is; urea is not.

• C. Little net water shift occurs, because urea crosses cell membranes freely and is an ineffective osmole (correct)

  Correct. Urea contributes to osmolality but not to tonicity, so cells keep their volume.

• D. Cells swell, because urea is trapped within the extracellular fluid

  Urea is not confined to the extracellular fluid; it distributes across total body water, so cells do not swell.

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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