The impermeability of the terminal part of the descending thin limb and the entire thin ascending limb in the inner medulla to water is noteworthy because AQP1 is not expressed in these nephron segments. However, these segments do express the Cl− channel CLC-K1, which facilitates the reabsorption of Cl− along with Na+ through the paracellular pathway. This passive reabsorption of NaCl without concurrent water reabsorption initiates the process of diluting the tubular fluid.

Furthermore, the thick ascending limb of the loop of Henle is also impermeable to water and actively reabsorbs NaCl from the tubular fluid, leading to further dilution. This segment is often referred to as the diluting segment of the kidney due to the extent of dilution. The fluid exiting the thick ascending limb is hypoosmotic compared to plasma.

In the distal tubule and cortical part of the collecting duct, there is active reabsorption of NaCl. In the absence or low levels of AVP, these segments are not permeable to water because AQP2 is not present in the apical membrane of the cells. As a result, the osmolality of the tubule fluid in these segments is further reduced since NaCl is reabsorbed without water. Consequently, the fluid leaving the cortical part of the collecting duct is hypoosmotic compared to plasma.

The medullary collecting duct also actively reabsorbs NaCl. Even without AVP, this segment has slight water permeability, leading to some water reabsorption.

The urine can have an osmolality as low as approximately 50 mOsm/kg H2O and contains low concentrations of NaCl. The volume of urine excreted can be as high as 18 L/day, which is approximately 10% of the glomerular filtration rate (GFR). Next, we will consider how the kidneys excrete concentrated urine (antidiuresis) when plasma osmolality and plasma AVP levels are high. The following numbers correspond to those circled in Fig. 35.7B and are similar to the steps involved in the production of dilute urine. It is important to note that while the reabsorption of NaCl by the ascending thin and thick limbs of the loop of Henle dilutes the tubular fluid, the reabsorbed NaCl accumulates in the medullary interstitium and increases its osmolality.

Accumulation of NaCl in the medullary interstitium is crucial for the production of urine that is hyperosmotic to plasma because it provides the osmotic driving force for water reabsorption by the medullary collecting duct. As mentioned earlier, AVP stimulates NaCl reabsorption by the thick ascending limb of Henle's loop. This is believed to maintain the medullary interstitial gradient when water is being added to this compartment from the medullary collecting duct, which would otherwise dissipate the gradient.
In the inner medulla the terminal portion of the descending thin limb and all of the thin ascending limb is impermeable to water. (NOTE: AQP1 is not expressed.) These same nephron segments express the Cl− channel CLC-K1, which mediates Cl− reabsorption, with Na+ following via the paracellular pathway. This passive reabsorption of NaCl without concomitant water reabsorption begins the process of diluting the tubular fluid.
4.
The thick ascending limb of the loop of Henle is also impermeable to water and actively reabsorbs NaCl from the tubular fluid and thereby dilutes it further (see ). Dilution occurs to such a degree that this segment is often referred to as the diluting segment of the kidney. Fluid leaving the thick ascending limb is hypoosmotic with respect to plasma (see
Fig. 35.6 5.
The distal tubule and cortical portion of the collecting duct actively reabsorb NaCl. In the absence of AVP these segments are not permeable to water (i.e., AQP2 is not present in the apical membrane of the cells). Thus when AVP is absent or present at low levels (i.e., decreased plasma osmolality), the osmolality of tubule fluid in these segments is reduced further because NaCl is reabsorbed without water. Under this condition, fluid leaving the cortical portion of the collecting duct is hypoosmotic with respect to plasma (see
Fig. 35.6 6.
The medullary collecting duct actively reabsorbs NaCl. Even in the absence of AVP, this segment is slightly permeable to water and some water is reabsorbed.
7.
The urine has an osmolality as low as approximately 50 mOsm/kg H2O and contains low concentrations of NaCl. The volume of urine excreted can be as much as 18 L/day, or approximately 10% of the glomerular filtration rate (GFR). Next, how the kidneys excrete concentrated urine (antidiuresis) when plasma osmolality and plasma AVP levels are high is considered. The following numbers refer to those encircled in
Fig. 35.7B : 1–4. These steps are similar to those for production of dilute urine. An important point in understanding how a concentrated urine is produced is to recognize that while reabsorption of NaCl by the ascending thin and thick limbs of the loop of Henle dilutes the tubular fluid, the reabsorbed NaCl accumulates in the medullary interstitium and raises the osmolality of this compartment.
• Fig. 35.7 Schematic of nephron segments involved in urine dilution and concentration. Henle’s loops of juxtamedullary nephrons are shown. A, Mechanism for excretion of dilute urine (water diuresis). AVP) is absent and the collecting duct is essentially impermeable to water. Note also that during a water diuresis the osmolality of the medullary interstitium is reduced as a result of increased vasa recta blood flow and entry of some urea into the medullary collecting duct. B, Mechanism for excretion of a concentrated urine (antidiuresis). Plasma AVP levels are maximal and the collecting duct is highly permeable to water. Under this condition the medullary interstitial gradient is maximal. See text for details.
Accumulation of NaCl in the medullary interstitium is crucial for production of urine hyperosmotic to plasma, because it provides the osmotic driving force for water reabsorption by the medullary collecting duct. As already noted, AVP stimulates NaCl reabsorption by the thick ascending limb of Henle’s loop. This is thought to maintain the medullary interstitial gradient at a time when water is being added to this compartment from the medullary collecting duct, which would tend to dissipate the gradient.
5.