Correct Statement About Loop of Henle Permeability

Which statement about the loop of Henle is correct?

Correct Answer: Both statements are correct

The loop of Henle has distinct permeability properties in its two limbs that are essential for the countercurrent multiplication mechanism of urine concentration.

Descending Limb Properties

• Permeable to water — water exits the tubule by osmosis into the hypertonic medullary interstitium
• Impermeable to solutes — NaCl and urea do not cross the descending limb wall
• Net effect: Tubular fluid becomes progressively more concentrated (hyperosmotic) as it descends

Ascending Limb Properties (Thin + Thick)

• Permeable to solutes — NaCl is reabsorbed (passively in thin limb, actively via NKCC2 in thick limb)
• Impermeable to water — water cannot follow the solutes; tubular fluid becomes dilute
• Net effect: Tubular fluid becomes progressively less concentrated (hypoosmotic) as it ascends

Why This Matters: Countercurrent Multiplication

These opposing permeability properties create the medullary osmotic gradient (300–1200 mOsm/kg) needed to concentrate urine in the collecting duct when ADH is present. The hairpin loop arrangement multiplies a small osmotic difference into a large gradient along the medulla.