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Constant is maintained through flux

Revised 17.09.2026 for accuracy — several statements on aquaporin regulation and renal reabsorption fractions have been refined.

In continually returning most of the filtrate to the blood, the kidney helps maintain body fluid and electrolyte balance within a normal range.

The kidney filters the blood. Most of the ultra-filtrate is returned to the blood — only excess salt, excess water, and metabolic waste products (urea, creatinine, and others) are ultimately removed.

Excess salt is removed by reducing the reabsorption of salts from the ultra-filtrate — the kidney can, in that way, make a dilute urine out of the ultra-filtrate. The kidney can also reabsorb much of the water, forming a concentrated urine out of the same ultra-filtrate. By adjusting these two mechanisms, the kidney can dilute or concentrate the ultra-filtrate to form (and excrete) either a dilute or a concentrated urine.

The kidney can concentrate the ultra-filtrate from around 285 mOsmol/kg (the tonicity of plasma) up to roughly 1200 mOsmol/kg in maximally concentrated urine. Given a typical daily solute load of 500–1000 mOsmol, this sets the minimum (“obligate”) urine volume at around 500 mL per day.

Anatomy: cortical vs juxta-medullary nephrons

The kidney uses counter-current exchange in the medulla to concentrate urine. This mechanism arises from the pairing of the loop of Henle with the vasa recta, and is a feature specifically of juxtamedullary nephrons. Unlike the cortical nephron, the juxtamedullary nephron has its glomerulus sitting at the cortico-medullary junction, with its tubule looping deep into the medulla — these looping tubules make up much of the tissue of the renal pyramids.

The tubule runs as a hairpin loop alongside the vasa recta, maximising exchange both with the plasma (via the vasa recta) and with the surrounding renal interstitium. Three fluid compartments are therefore involved in the counter-current exchange mechanism: the tubular fluid, the plasma (vasa recta) fluid, and the interstitial fluid.

It helps to think of the renal ultra-filtrate as the portion of plasma routed through the medullary interstitium — indirectly exposing it to the electrolyte concentration and tonicity of that interstitium as it passes. The tubule epithelium is variably permeable to different solutes along its length, and that permeability can itself be actively modified — for instance by inserting additional channels into the epithelium. The clearest example is the collecting duct, where epithelial permeability to water (and urea) can be dramatically and rapidly altered under hormonal control (see ADH, below).

The Renal Medulla – Interstitium and Counter-current multiplier

The inner workings of the kidney reflect the anatomy of:

• medullary collecting ducts
• loops of Henle
• vasa recta
• interstitium

The counter-current multiplier is the renal concentrating mechanism. The kidney creates a concentration gradient along the medullary interstitium, running from the cortico-medullary junction down to the renal papilla. Ultra-filtrate passing through this gradient has water drawn out of the tubule and into the interstitium, concentrating the urine — this is the main function of the loop of Henle. (The “thin” and “thick” segments are named for their simple squamous and simple columnar/cuboidal epithelium, respectively.)

Because of the counter-current multiplier effect, an osmotic gradient is set up along the kidney’s cortico-papillary axis, and the ultra-filtrate is passed through this gradient to achieve its final concentration.

In the medulla, tubular loops are stacked side by side with little intervening interstitium, allowing efficient solute transfer between filtrate and interstitium.

 histology — loop of Henle thin segment vs ascending thick segment

The thin descending limb, thick ascending limb, and collecting duct each have distinct permeabilities: the descending limb to water (by passive diffusion), the ascending limb to ions, and the collecting duct to water and urea.

The collecting duct is where final water salvage occurs. Under basal conditions, passive diffusion of water through AQP-1 channels means the thin limb is considerably more water-permeable than the collecting duct — most tubular fluid water is already reabsorbed by the time filtrate reaches the thick ascending limb.

In the presence of anti-diuretic hormone (ADH), the collecting duct’s water permeability increases substantially. The main mechanism is rapid trafficking of aquaporin-2 (AQP-2) channels into the apical membrane of the principal cells; water then exits the cell into the interstitium via basolateral channels, chiefly AQP-3 and AQP-4, which are more constitutively expressed rather than acutely regulated in the same way as AQP-2.* ADH release itself reflects body volume status — a low-volume state stimulates ADH release, and the collecting duct correspondingly ramps up water reabsorption to concentrate the urine.

*Humans express 13 known aquaporins.

Overall, the kidney produces around 180 L of ultra-filtrate per day. The large majority of this is reabsorbed well before the collecting duct — the proximal tubule alone accounts for roughly two-thirds, with the loop of Henle and distal tubule reabsorbing most of the remainder. The collecting duct is left to fine-tune reabsorption of only the last small fraction, but it’s this final adjustment — under ADH control — that determines whether the urine that’s ultimately excreted is dilute or concentrated.

Works Cited

1. Amerman, Erin C. (Ed.), “The Urinary System,” in Human Anatomy and Physiology, 2nd Edn. (Pearson, 2018), 944. https://www.pearson.com/content/dam/one-dot-com/one-dot-com/us/en/higher-ed/en/products-services/course-products/amerman-1e-info/pdf/amerman-sample-chapter24.pdf.