Nephron loop (loop of Henle)
Ansa nephroni
Also known as: loop of Henle, Henle loop, ansa nephroni
The U-shaped part of the nephron between the PCT and DCT that dips into the medulla — descending limb, (thin ascending limb) and thick ascending limb — reabsorbing ≈25% of filtered NaCl and ≈15–20% of water and generating the medullary osmotic gradient.
Overview
Strictly, the loop comprises the straight part of the PCT, the thin descending limb, the thin ascending limb (long loops only), and the thick ascending limb (TAL), which ends at the macula densa. Clinically "loop of Henle" usually refers to the thin limbs plus the TAL.
The two limbs have opposite permeabilities, which is the key to their function. The descending limb is permeable to water (AQP1) but has low NaCl permeability, so fluid entering at ≈300 mOsm/kg loses water to the hypertonic interstitium and is concentrated to as much as ≈1200 mOsm/kg at the bend of long loops. The ascending limb is impermeable to water; NaCl leaves passively in the thin part and is pumped out actively by NKCC2 in the thick part, so fluid reaching the DCT is hypo-osmotic (≈100 mOsm/kg) regardless of hydration state. This arrangement — two parallel limbs with opposite flow and active transport in one — is the countercurrent multiplier that builds the corticomedullary gradient used by the collecting duct to concentrate urine.
Overall the loop reabsorbs ≈25% of filtered Na⁺, Cl⁻ and K⁺, ≈20–25% of Ca²⁺, ≈60–70% of Mg²⁺ (TAL) and ≈15–20% of filtered water (descending limb).
- Location
Medulla (and medullary rays); short loops turn in the outer medulla, long loops in the inner medulla.
Parts & subdivisions
- Water-permeable; tubular fluid concentrated.
- Water-impermeable; passive NaCl exit (long loops only).
- NKCC2; diluting segment; ends at the macula densa.
Anatomy
- Arterial supply
Vasa recta (juxtamedullary nephrons); peritubular capillaries for the cortical parts.
Function
- Generate the medullary osmotic gradient (countercurrent multiplication)
- Reabsorb ≈25% of filtered NaCl and ≈15–20% of water
- Dilute the tubular fluid (≈100 mOsm/kg leaving the TAL)
- Reabsorb Ca²⁺ and most filtered Mg²⁺
Clinical significance
Loop diuretics
Furosemide, bumetanide and torasemide inhibit NKCC2 in the TAL — the most potent diuretics (up to ≈25% of filtered Na⁺). Side effects: hypokalaemia, metabolic alkalosis, hypocalcaemia, hypomagnesaemia, hypovolaemia, ototoxicity, hyperuricaemia. Used in pulmonary oedema, heart failure and hypercalcaemia.
Compiled from standard anatomy and physiology references — see the Urinary System sources. For education only; not medical advice.