Thin descending limb of Henle
Loop of Henle · Water only AQP1 Mannitol
Water-permeable, salt-impermeable segment that dives from cortex toward papilla, equilibrating with the increasingly hypertonic medullary interstitium so that tubular fluid is concentrated to ~1200 mOsm/kg at the bend of long loops.
Traced from the start
Detail
- Reabsorbs
- Water (~15-20% of the filtered load) by osmosis via AQP1; essentially no active NaCl transport
- Permeability
- High to water, low to NaCl; urea enters via UT-A2 as part of urea recycling
- Result
- Fluid osmolality rises from ~300 to up to ~1200 mOsm/kg at the hairpin in juxtamedullary nephrons (only ~15% of nephrons have long loops)
- Hormone
- None directly
- Diuretic
- Osmotic diuretics (mannitol; glucose in hyperglycaemia): trapped luminal solute holds water in the proximal tubule and descending limb, reducing water and secondarily Na+ reabsorption
When it goes wrong
Mannitol infusion
Initial ECF expansion before diuresis (can precipitate pulmonary oedema in heart failure), then hypernatraemia and volume depletion; used to lower intracranial pressure
Medullary washout after loop diuretics or prolonged high-flow states
Less gradient for the descending limb to equilibrate against, so urine cannot be concentrated above ~300 mOsm/kg (isosthenuria)
Practise this structure
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