CorpusHuman Anatomy

Nephron — Juxtamedullary

Tubule, renal microcirculation and the medullary osmotic gradient

A juxtamedullary nephron (≈15 % of nephrons) — the type with a long loop dipping deep into the inner medulla that builds the corticomedullary osmotic gradient. Blood reaches the glomerulus through the afferent arteriole and leaves through the efferent arteriole, which feeds the peritubular capillaries around cortical tubules and the hairpin vasa recta alongside the loop. Filtrate flows (animated particles) from Bowman's capsule through the proximal tubule, thin descending limb, thin and thick ascending limbs, back to its own vascular pole at the juxtaglomerular apparatus, then through the distal convoluted tubule into a collecting duct, where it becomes urine.

The violet background shades the interstitial osmolality, rising from ≈300 mOsm/kg at the corticomedullary junction to ≈1200 mOsm/kg at the papillary tip. Italic numbers give the osmolality of the tubular fluid itself (isosmotic 300 in the proximal tubule, ≈1200 at the bend, ≈100 leaving the thick ascending limb, ≈50–1200 in final urine depending on ADH). Arrows show water (blue) leaving the descending limb and ADH-responsive collecting duct, NaCl (orange) leaving the water-impermeable ascending limb, and urea (green) recycled from the inner medullary collecting duct.

Step-by-step

  1. 1 · Glomerular filtration

    Blood enters the glomerular capillaries from the afferent arteriole at high hydrostatic pressure (≈55–60 mmHg). Opposed by Bowman’s space pressure (≈15–18 mmHg) and plasma oncotic pressure (≈30 mmHg), a net filtration pressure of ≈10 mmHg drives a protein-free ultrafiltrate across fenestrated endothelium, basement membrane and podocyte slit diaphragms. GFR ≈125 mL/min (≈180 L/day); filtration fraction ≈20 %. The efferent arteriole carries blood on.

  2. 2 · Proximal tubule: bulk reabsorption

    The brush-bordered proximal tubule reabsorbs ≈65 % of filtered Na⁺ and water isosmotically (fluid stays ≈300 mOsm/kg), essentially all glucose (SGLT2/SGLT1) and amino acids, ≈80 % of HCO₃⁻ (via Na⁺/H⁺ exchange and carbonic anhydrase), most phosphate, K⁺ and ≈50 % of urea. It secretes organic anions and cations (e.g. PAH, penicillin, creatinine) and produces NH₄⁺. Reabsorbate is taken up by the peritubular capillaries.

  3. 3 · Thin descending limb: water out

    The thin descending limb is highly permeable to water (aquaporin-1) but relatively impermeable to NaCl. As it plunges into an increasingly hyperosmotic interstitium, water leaves by osmosis, so the tubular fluid becomes concentrated, reaching ≈1200 mOsm/kg at the hairpin bend.

  4. 4 · Ascending limbs: salt out, water stays

    Both ascending limbs are impermeable to water. The thin ascending limb loses NaCl passively; the thick ascending limb actively reabsorbs ≈25 % of filtered Na⁺ via the apical Na⁺–K⁺–2Cl⁻ cotransporter (NKCC2, the target of loop diuretics), plus Ca²⁺ and Mg²⁺ paracellularly. Removing solute without water dilutes the fluid to ≈100 mOsm/kg (the diluting segment) and powers the countercurrent multiplier.

  5. 5 · Juxtaglomerular apparatus

    The thick ascending limb returns to its own glomerulus. Macula densa cells sense luminal NaCl via NKCC2: high delivery triggers tubuloglomerular feedback (adenosine constricts the afferent arteriole, lowering GFR); low delivery, low afferent pressure or β₁-sympathetic stimulation makes the granular juxtaglomerular cells release renin, starting the renin–angiotensin–aldosterone system.

  6. 6 · Distal convoluted tubule

    The early DCT reabsorbs ≈5 % of filtered Na⁺ through the thiazide-sensitive Na⁺–Cl⁻ cotransporter (NCC). It is impermeable to water, so the fluid is diluted further. It is the main site of regulated Ca²⁺ reabsorption, increased by parathyroid hormone (TRPV5 channels) and also by thiazides.

  7. 7 · Collecting duct: final adjustment

    Principal cells reabsorb Na⁺ through ENaC and secrete K⁺ through ROMK, both stimulated by aldosterone; intercalated cells secrete H⁺ (type A) or HCO₃⁻ (type B). ADH (V₂ receptors) inserts aquaporin-2, so water leaves into the hyperosmotic medulla: urine reaches up to ≈1200 mOsm/kg in antidiuresis, but can be as dilute as ≈50 mOsm/kg without ADH. Urea leaving the inner medullary duct (UT-A1/3) adds to the medullary gradient.

  8. 8 · Vasa recta: countercurrent exchange

    Efferent arterioles of juxtamedullary glomeruli descend as the vasa recta. Their hairpin, low-flow arrangement lets solutes and water exchange passively between descending and ascending limbs, so blood removes the reabsorbed water without washing out the medullary gradient. Ascending vasa recta drain into the arcuate vein.

Labelled parts