Formation of Dilute Urine

Juxtamedullary nephrons produce dilute urine during high fluid intake because antidiuretic hormone (ADH) levels decrease, making the late distal convoluted tubule and collecting ducts impermeable to water. Although ions continue to be reabsorbed, water remains in the tubular lumen and is excreted as dilute urine, preventing fluid overload.

In the proximal convoluted tubule, filtrate osmolarity remains isosmotic with the surrounding renal cortex because both solutes and water are reabsorbed equally. This balanced reabsorption maintains the osmolarity of the filtrate at the same level as the interstitial fluid in the cortex.

The ascending limb actively transports ions like sodium, potassium, and chloride into the interstitial fluid but is impermeable to water. As ions leave the filtrate without water following, the filtrate becomes progressively more dilute, reducing its osmolarity as it moves toward the distal convoluted tubule.

The descending limb is permeable to water but impermeable to ions. As the filtrate enters the renal medulla where interstitial fluid osmolarity is significantly higher, water passively exits the descending limb, increasing the filtrate's osmolarity and establishing the osmotic gradient necessary for subsequent ion reabsorption.

The early distal convoluted tubule reabsorbs ions but remains impermeable to water, similar to the ascending limb. This selective ion reabsorption without water reabsorption further reduces the filtrate's osmolarity, continuing the dilution process that began in the ascending limb of the nephron loop.

During overhydration, low ADH levels decrease the permeability of the late distal convoluted tubule and collecting ducts to water. This prevents water reabsorption despite continued ion reabsorption, allowing excess water to remain in the tubular lumen and be excreted as dilute urine.

The osmotic gradient between the renal medulla's high-osmolarity interstitial fluid and the filtrate in the descending limb drives passive water movement out of the tubule. This water reabsorption concentrates the filtrate while maintaining the medullary osmotic gradient essential for the countercurrent multiplier system.