Renal Filtration and Electrolyte Balance · USMLE Step 1

Which principle explains why water follows sodium reabsorption along much of the nephron, helping to concentrate urine and regulate volume?

  1. Active transport of water by aquaporins only
  2. Osmosis following solute and sodium reabsorption
  3. Hydrostatic pressure within Bowman's capsule space
  4. Adjustment of the glomerular filtration fraction
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Correct answer: Osmosis following solute and sodium reabsorption

As sodium (and other solutes) is reabsorbed from the filtrate, osmotic gradients are generated that draw water passively across the tubular epithelium, aiding water reabsorption and volume regulation.

Difficulty: Medium Question 14 of 20

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