The capillary networks known as the vasa recta are highly permeable to solute and water, providing blood supply to the medulla. Similar to the loop of Henle, the vasa recta form hairpin loops within the medulla. These capillaries play a crucial role in not only delivering nutrients and oxygen to the medullary nephron segments, but also in removing excess water and solute that continuously accumulates in the medullary interstitium due to the nephron segments. On a typical diet, the kidneys need to excrete 450 mmol/day of urea. If the maximum concentration of urea in the urine is 600 mmol/L, this amount can be excreted in less than 1 liter of urine. However, if the maximum concentration of urea in the urine is lower due to a decrease in the medullary interstitial fluid, a larger volume of urine would be required to excrete the 450 mmol/day of urea (e.g., 2.25 liters of urine would be needed if the maximum concentration of urea in the urine is only 200 mM).

The vasa recta's ability to maintain the medullary interstitial gradient depends on blood flow. A significant increase in vasa recta blood flow results in the dissipation of the medullary gradient, causing osmoles to be washed out from the medullary interstitium. Conversely, reduced blood flow leads to a decrease in oxygen delivery to the nephron segments in the medulla. As salt and solute transport by the nephron segments in the medulla requires oxygen and ATP, reduced medullary blood flow impairs salt and solute transport. Consequently, the medullary interstitial osmotic gradient cannot be maintained.

Assessing renal water handling involves measuring urine osmolality and volume. The range of urine osmolality varies from 50 to 1200 mOsm/kg H2O, while the corresponding range in urine volume is from 18 liters to as little as 0.5 liters per day. These ranges are not fixed and vary among individuals. They can also be influenced by disease processes and the amount of solute that needs to be excreted by the kidneys.

The kidneys' ability to dilute or concentrate urine relies on the separation of solute and water through the countercurrent multiplication process. This separation generates solute-free water, which is either excreted from the body when urine is dilute or conserved when urine is concentrated.

To maximally excrete solute-free water (i.e., 18 liters per day), the following conditions must be met:
1. AVP (anti-diuretic hormone) must be absent, as the absence of AVP prevents significant water reabsorption in the collecting duct.
2. The tubular structures responsible for separating solute from water must function properly. In the absence of AVP, the following nephron segments can dilute the luminal fluid: ascending thin limb of Henle's loop and thick ascending limb of Henle's loop. Among these segments, the thick ascending limb is the most important in terms of quantitatively separating solute and water.
3. An adequate amount of tubular fluid must be delivered to the aforementioned nephron sites for optimal separation of solute and water. Factors that reduce delivery, such as decreased glomerular filtration rate (GFR) or increased reabsorption in the proximal tubule, impair the kidneys' ability to maximally excrete solute-free water. Similar requirements apply to water conservation by the kidneys. For the kidneys to conserve water maximally (6-8 liters per day), an adequate amount of tubular fluid must be delivered to the nephron segments responsible for separating solute from water, with the most crucial segment being the...
The vasa recta, the capillary networks that supply blood to the medulla, are highly permeable to solute and water. As with the loop of Henle, the vasa recta form a parallel set of hairpin loops within the medulla (see ). Not only do the vasa recta bring nutrients and oxygen to the medullary nephron segments, but more importantly they also remove the excess water and solute that is continuously added to the medullary interstitium by these nephron seg eOn a typical diet the kidneys must excrete 450 mmol/day of urea. At a maximal urine [urea] of 600 mmol/L this amount of urea can be excreted in less than 1 L of urine. However, if the maximal urine [urea] is reduced because of a decrease in the medullary interstitial fluid [urea], a larger urine volume would be needed to excrete the 450 mmol/day of urea (e.g., 2.25 L of urine would be required if the maximal urine [urea] was only 200 mM).
CHAPTER 35 Control of Body Fluid Osmolality and Volume ments. The ability of the vasa recta to maintain the medullary interstitial gradient is flow dependent. A substantial increase in vasa recta blood flow dissipates the medullary gradient (i.e., washout of osmoles from the medullary interstitium). Alternatively, reduced blood flow reduces oxygen delivery to the nephron segments within the medulla. Because transport of salt and other solutes requires oxygen and ATP, reduced medullary blood flow decreases salt and solute transport by nephron segments in the medulla. As a result the medullary interstitial osmotic gradient cannot be maintained.
Assessment of Renal Diluting and Concentrating Ability
Assessment of renal water handling includes measurements of urine osmolality and the volume of urine excreted. The range of urine osmolality is from 50 to 1200 mOsm/kg H2O. The corresponding range in urine volume is 18 L to as little as 0.5 L/day. These ranges are not fixed and vary from individual to individual. They can also be affected by disease processes, and as noted previously they are dependent on the amount of solute the kidneys must also excrete.
The ability of the kidneys to dilute or concentrate urine requires the separation of solute and water (i.e., the single effect of the countercurrent multiplication process). This separation of solute and water in essence generates a volume of water that is “free of solute.” When urine is dilute, solute-free water is excreted from the body. When urine is concentrated, solute-free water is returned to the body (i.e., conserved).
For the kidneys to maximally excrete solute-free water (i.e., 18 L/day) the following conditions must be met: 1.
AVP must be absent; without it the collecting duct does not reabsorb a significant amount of water.
2.
The tubular structures that separate solute from water (i.e., dilute the tubule fluid) must function normally. In the absence of AVP the following nephron segments can dilute the luminal fluid: ascending thin limb of Henle’s loop thick ascending limb of Henle’s loop
Because of its high transport rate, the thick ascending limb is quantitatively the most important nephron segment involved in the separation of solute and water.
3. An adequate amount of tubular fluid must be delivered to the aforementioned nephron sites for maximal separation of solute and water. Factors that reduce delivery (e.g., decreased GFR or enhanced proximal tubule reabsorption) impair the kidneys’ ability to maximally excrete solute-free water. Similar requirements also apply to conservation of water by the kidneys. For the kidneys to conserve water maximally (6–8 L/day) the following conditions must be met: 1. An adequate amount of tubular fluid must be delivered to those nephron segments that separate solute from water; the most important segment in this regard is the