URINE CONCENTRATION AND DILUTION AND OVERVIEW OF WATER HANDLING
In normal kidneys more than 180 liters of ﬂuid are ﬁltered into the nephrons each day, but nearly all of it is reabsorbed into the peritubular circulation.
Tight junctions form a watertight seal between tubular epithelial cells throughout most of the nephron. Thus, water reabsorption occurs primarily through a transcellular route, requiring specialized channels known as aquaporins (AQPs) in both the apical and basolateral compartments of the plasma membrane.
Because aquaporins are channels, and not pumps, the reabsorption of water is a passive process, dependent on osmotic pressure from solutes concentrated in the sur-rounding interstitium.
In each tubular segment, the reabsorption of water can be greater than, less than, or equal to the reabsorption of solutes. As a result, urine becomes more concentrated as it passes through some segments and more diluted as it passes through others. The ﬁnal concentration of excreted urine is determined in the collecting duct, which reﬂects not only the fact that this segment is located at the end of the nephron, but also that it reabsorbs water at a variable rate based on hormonal input.
Proximal Tubule. The proximal tubule reabsorbs two thirds of the ﬁltered water. There is a large gradient for water reabsorption from this segment because of the high rate of solute reabsorption. As solute begins to accumulate in the interstitium, water crosses from the tubular lumen to the interstitium through AQP-1 channels in both the apical and basolateral plasma membranes.
Because water reabsorption from the proximal tubule is directly dependent on the rate of solute reabsorption, and because AQP-1 channels are always present, the ﬁltrate remains iso-osmotic to plasma as it passes through this segment.
Descending Thin Limb. The descending thin limb reabsorbs an additional fraction of the ﬁltered water. There is a large gradient for water reabsorption from this segment even though it reabsorbs only a small amount of solute. This gradient reﬂects the high rates of reabsorption from the thick ascending limb, which is adjacent to the ascending thin limb and adds solute to its surrounding interstitium. As in the proximal tubule, water crosses the tubular epithelium through AQP-1 channels.
As described on Plate 1-24, the descending thin limbs of short-looped (cortical) and long-looped (juxtamedullary) nephrons differ not only in length but also in cellular composition. In short-looped nephrons, the descending thin limb consists of type I cells, whereas in long-looped nephrons, it consists of type II cells in the outer medulla and type III cells in the inner medulla. Type I and II cells are more permeable to water than type III cells. Thus, in long-looped nephrons, water reabsorption from the descending thin limb decreases near the inner medulla.
Because water reabsorption exceeds solute reabsorption in the descending thin limb, tubular ﬂuid becomes more concentrated. This process, however, is not under tight control. As a result, the descending thin limb does not have a major role in determining the ﬁnal concentration of excreted urine.
Ascending Thin Limb and Thick Ascending Limb. The ascending thin limb (found only in long-looped nephrons) and thick ascending limb do not contain aquaporin channels and are therefore impermeable to water. The extensive reabsorption of solutes from these segments, however, dilutes tubular ﬂuid and establishes a concentration gradient for water reabsorption from adjacent segments, such as the descending thin limb and collecting duct.
Because the dilution process in the ascending limb is not under tight control, this segment does not have a major role in determining the ﬁnal concentration of excreted urine.
Distal Convoluted Tubule. Like the thick ascending limb, the distal convoluted tubule reabsorbs solutes but is impermeable to water. Therefore, this segment dilutes tubular ﬂuid but, for the same reasons as the thick ascending limb, does not have a major role in determining the ﬁnal concentration of excreted urine. Connecting Tubule and Collecting Duct. The connecting tubule and collecting duct reabsorb a variable volume of ﬁltered water, which determines the ﬁnal concentration of excreted urine.
By reabsorbing more or less free water from the urine, these segments can dilute or concentrate plasma, helping to offset the changes in osmolality that result from inconsistent intake of water and salt over the course of each day. The hormone that controls water reabsorption is known as antidiuretic hormone (ADH, or vasopressin).
In response to increases in plasma osmolality, ADH is released from the posterior pituitary. In the connecting tubule and collecting duct this hormone causes vesicles containing AQP-2 channels to fuse with the apical plasma membrane of principal cells. Since AQP-4 channels are always present in the basolateral plasma membrane of these cells, the insertion of AQP-2 channels is sufﬁcient to cause a dramatic increase in water reabsorption.
Because of the countercurrent multiplier system, there is a strong gradient for water reabsorption from the collecting duct that increases in strength toward the papillae. Because water reabsorption is a passive process, the maximum achievable urine concentration is equal to the peak osmolality in the medullary interstitium, about 1200 mOsm/kg H2O. Such concentrations are only achievable in long-looped nephrons, however, because short-looped nephrons do not reach the inner medulla.
In addition to its direct effects on aquaporin channels, ADH has several actions that enhance the countercurrent system and thus increase the gradient for water reabsorption. In particular, this hormone increases solute reabsorption from the thick ascending limb, constricts vasa recta capillaries to reduce solute washout, and increases urea reabsorption from the inner medullary collecting duct. Some of the urea that drifts toward the cortex is secreted back into more proximal segments of the renal tubules so that it can be deposited again in the inner medulla.
In response to decreases in plasma osmolality, ADH release is inhibited, and AQP-2 channels are consequently endocytosed. The lack of water reabsorption from the collecting duct, coupled with the ongoing reabsorption of sodium from this segment, dilutes the urine to a minimum osmolality of 50 mOsm/kg H2O.
Over the course of several hours, variable input from the ADH system leads to accumulation of urine in the bladder that has an osmolality between 50 and 1200 mOsm/kg H2O. In patients with abnormal serum sodium concentrations, measurement of the urine osmolality can indicate whether the defect lies in the urine concentration process or elsewhere.