Osmolality Along Loop of Henle

Which part of loop of Henle has the highest and lowest osmolality respectively?

Correct Answer: Hairpin loop (highest) & Ascending thick limb (lowest)

The osmolality of tubular fluid changes dramatically along the loop of Henle due to differential permeability to water and solutes.

Osmolality Profile Along Loop of Henle

• Start of descending limb (cortex): ~300 mOsm/kg (isotonic with plasma)
• Descending limb → hairpin loop: Osmolality progressively INCREASES as water exits into the hypertonic medulla
• Hairpin (tip) of loop: ~1200 mOsm/kg — HIGHEST osmolality
• Ascending limb (thin → thick): Osmolality progressively DECREASES as NaCl is reabsorbed without water following
• End of thick ascending limb (cortex): ~100–150 mOsm/kg — LOWEST osmolality (hypo-osmotic/dilute)

Memory Aid

Think of a U-shape: as you go DOWN, concentration goes UP (water leaves, fluid concentrates). As you go UP, concentration goes DOWN (NaCl leaves but water can't follow, so fluid dilutes). The deepest point = highest concentration = hairpin loop.

Clinical Relevance

This concentration gradient along the loop is the basis of the countercurrent multiplication mechanism that allows mammals to produce concentrated urine. Species with longer loops (dog, cat) achieve higher maximum medullary osmolality.