6: Basic Renal Processes for Sodium, Chloride, and Water

• ▸ List approximate percentages of sodium reabsorbed in major tubular segments.

• ▸ List approximate percentages of water reabsorbed in major tubular segments.

• ▸ Describe proximal tubule sodium reabsorption, including the functions of the apical membrane sodium entry mechanisms and the basolateral Na-K-ATPase.

• ▸ Explain why chloride reabsorption is coupled with sodium reabsorption, and list the major pathways of proximal tubule chloride reabsorption.

• ▸ State the maximum and minimum values of urine osmolality.

• ▸ Define osmotic diuresis and water diuresis.

• ▸ Explain why there is always an obligatory water loss.

• ▸ Describe the handling of sodium by the descending and ascending limbs, distal tubule, and collecting-duct system.

• ▸ Describe the role of sodium-potassium-2 chloride symporters in the thick ascending limb.

• ▸ Describe the handling of water by descending and ascending limbs, distal tubule, and collecting-duct system.

• ▸ Describe the process of “separating salt from water” and why this is required to excrete either concentrated or dilute urine.

• ▸ Describe how antidiuretic hormone affects water and urea reabsorption.

• ▸ Describe the characteristics of the medullary osmotic gradient.

• ▸ Explain the role of the thick ascending limb, urea recycling, and medullary blood flow in generating the medullary osmotic gradient.

• ▸ State why the medullary osmotic gradient is partially “washed out” during a water diuresis.

This and Chapter 7 are devoted entirely to the renal handling of sodium, chloride, and water. Sodium and chloride are crucial substances because they account for most of the osmotic content of the extracellular fluid while water constitutes the major part of the body volume. And water is the solvent for all dissolved solutes.

As described in Chapter 7, these substances play a huge role in the function of the cardiovascular system and are subject to complex regulation.

Body Fluid Compartments

Approximately 60% of the body weight is made up of water, which is distributed into various aqueous spaces in proportion to their osmotic content. The collective volume of all the cells in the body is called the intracellular fluid (ICF). It contains roughly two thirds of the body osmotic content, and therefore two thirds of the water. The remaining one third of the osmotic content and water is called the extracellular fluid (ECF). It is mostly interstitial fluid and blood plasma. Because of the ease with which water crosses most cell membranes (see Chapter 4) the ECF and ICF are in osmotic equilibrium. The total of the 2 volumes varies with gain and loss of water, whereas the relative proportion in each compartment is influenced by gain and loss of sodium. Additions or losses of sodium from the body are mostly to or from the ECF because the actions of cellular Na-K-ATPases prevent major changes in intracellular sodium concentration.¹ If the addition or loss of fluid is isotonic sodium, only the volume of the ECF is affected, but if the fluid is either hyper- or hypo-osmotic, both compartments change volume. These events are depicted in Figure 6–1. The addition of water alone expands both the ICF and ECF (indicated by the dashed lines). Adding sodium chloride without water does not change the total volume, but causes a shift of water from the ICF to the ECF in order to restore equality of osmolality between the 2 compartments.

Sodium, chloride, and water are all freely filterable at the renal corpuscle. They all undergo considerable tubular reabsorption, (usually more than 99%!), but normally no tubular secretion. Most of the renal ATP energy expended every day is used to accomplish this enormous reabsorptive task. In terms of transport mechanisms, the transport of water is the simplest. As we pointed out in Chapter 4, “water follows the osmoles.” Thus, much of the description for water transport really amounts to describing solute transport, taking into account the fact that in some regions of the kidney the epithelium has low water permeability, leaving water confined to the tubular lumen even though solute is removed. Transport of chloride involves more steps, but is often passive and, because of the constraints of electroneutrality, tied to the transport of sodium. Sodium transport is the most complicated. First, its transport is linked to the transport of many other substances, and second, its rates of transport at various places are subject to regulation by multiple controls. However, if we keep in mind the generalized model of epithelial transport developed in Chapter 4 (Figure 4–4) it is not difficult to grasp the key features of sodium transport.

The inputs, and therefore excretory rates, of sodium, chloride, and water vary over an extremely wide range. For example, some persons may ingest 20 to 25 g of sodium chloride per day, whereas a person on a low-salt diet may ingest only 0.05 g. The normal kidney can readily alter its excretion of salt over this range. Similarly, urinary water excretion can be varied from approximately 0.4 to 25 L/day, depending on whether one is lost in the desert or drinking excessive water for nonphysiological reasons.

Sodium Reabsorption

Table 6–1 is a balance sheet for sodium chloride. Clearly the major route of salt excretion from the body under normal circumstances is via the kidneys. The large amount excreted should not obscure the fact that nearly all the filtered sodium is reabsorbed. Table 6–2 summarizes the approximate quantitative contribution of each tubular segment to sodium reabsorption. In an individual with an average salt intake, the proximal tubule reabsorbs about 65% of the filtered sodium, the thin and thick ascending limbs of Henle's loop about 25%, and the distal convoluted tubule and collecting-duct system most of the remaining 10%, so that the final urine contains less than 1% of the total filtered sodium. As discussed in Chapter 7, reabsorption at several of these tubular sites is under physiological control by multiple signals, so that the exact amount of sodium excreted is homeostatically regulated. Because so much sodium is filtered, even a small percentage change in reabsorption results in a relatively large change in excretion.

In all nephron segments, the essential event for active transcellular sodium reabsorption is the primary active transport of sodium from cell to interstitial fluid by the Na-K-ATPase pumps in the basolateral membrane. These pumps keep the intracellular sodium concentration lower than in the surrounding media. In addition, the inside of the cell is negatively charged with respect to the lumen, and luminal sodium ions enter the cell passively, down both their concentration and electrical gradients.

Chloride Reabsorption

The tubular locations that reabsorb chloride and the percentages of filtered chloride reabsorbed by these segments are similar to those for sodium because of the constraints of electroneutrality (see Table 6–1). Any finite volume of fluid must contain equal amounts of anion and cation equivalents. One liter of normal filtrate contains 140 mEq of sodium and approximately 140 mEq of anions, mainly chloride (110 mEq) and bicarbonate (24 mEq). (We say “approximately” because there are other cations [eg, potassium and calcium] and anions [eg, sulfate and phosphate], but their contributions are much smaller than sodium, chloride, and bicarbonate.) If 65% of the sodium in 1 liter of filtrate is reabsorbed in the proximal tubule (0.65 × 140 = 91 mEq), then electroneutrality requires that 91 mEq of some combination of chloride and bicarbonate must also be reabsorbed in the proximal tubule to accompany this sodium. As described in Chapter 9, approximately 90% of the filtered bicarbonate is reabsorbed in the proximal tubule (0.9 × 24 ≈ 22). This leaves 91 – 22 = 69 mEq of chloride that must be reabsorbed in the proximal tubule. This is more than 60% of the filtered chloride and very similar to the fractional reabsorption of sodium. Later segments reabsorb almost all of the remaining 40%.

There is both passive paracellular chloride reabsorption as well as active transcellular reabsorption. In active transcellular chloride reabsorption, the critical transport step for chloride is usually from lumen to cell. The chloride transport process in the luminal membrane must go against the negative membrane potential that repels anions, and it must achieve a high enough intracellular chloride concentration to drive downhill chloride movement out of the cell across the basolateral membrane. Thus, luminal membrane chloride transporters serve essentially the same function for chloride that the basolateral membrane Na-K-ATPase pumps do for sodium: they use energy to move chloride uphill from lumen to cell against its electrochemical gradient.

Water Reabsorption

A balance sheet for total body water is given in Table 6–3. These are average values, which are subject to considerable variation. The 2 sources of body water are metabolically produced water, resulting largely from the oxidation of carbohydrates, and ingested water obtained from liquids and so-called solid food (eg, a rare steak is approximately 70% water). There are several sites from which water is always lost to the external environment: skin, lungs, gastrointestinal tract, and kidneys. Menstrual flow and, in lactating women, breast milk constitute 2 other potential sources of water loss in women.

The loss of water by evaporation from the cells of the skin and the lining of respiratory passageways is a continuous process, often referred to as insensible loss because people are unaware of its occurrence. Additional water evaporates from the skin during production of sweat. Fecal water loss is normally quite small but can be severe in diarrhea. Gastrointestinal loss can also be large during severe bouts of vomiting. Under conditions of normal hydration, the kidneys are, of course, the main route of water loss.

With a large water load, the renal response is to produce a large volume of very dilute urine (osmolality much lower than in blood plasma). In contrast, during a state of dehydration, the urine volume is low and very concentrated (ie, the urine osmolality is much greater than in blood plasma). That the urine osmolality is so variable brings us to a crucial aspect of renal function. Terrestrial animals must be able to independently control excretion of salt and water, because their ingestion and loss is not always linked (see Tables 6–1 and 6–3). To excrete water in excess of salt and vice versa (ie, produce a range of urine osmolalities), the kidneys must be able to separate the reabsorption of solute from the reabsorption of water, that is, to “separate salt from water,” a process described later in this chapter. Water reabsorption always occurs in the proximal tubule (65% of the filtered water), descending thin limb of Henle's loop (10%), and collecting-duct system (where the fractional reabsorption is highly variable). A comparison of water and sodium reabsorption (see Table 6–2) reveals several important points. First, sodium and water reabsorption occur in the proximal tubule to the same extent. Second, both are also reabsorbed in Henle's loop, but not in equal proportions. The part of the loop involved in water reabsorption is different than for sodium reabsorption, and the fraction of sodium reabsorbed by the loop as a whole is always greater than that of water (ie, the loop overall is a site where salt is reabsorbed and excess water is left in the lumen of the nephron: “separating salt from water”). By the time tubular fluid leaves the loop of Henle and enters the distal tubule, the loss of solute has typically decreased the osmolality to only one third of the plasma value. Third, sodium reabsorption, but not water reabsorption, occurs in the distal convoluted tubule. Fourth, both occur in the collecting-duct system, but the percentages of sodium and water reabsorbed in the collecting-duct system vary enormously depending on body conditions.

Water can cross the tubular epithelium by several routes. Small amounts can move by simple diffusion through the lipid bilayer, but not enough to be significant. The majority moves through aquaporins in plasma membranes of the tubular cells and through the tight junctions between the cells. The amount of water that moves for a given osmotic gradient and its route depends on the water permeability of the different cellular components. The basolateral membranes of all renal cells are quite permeable to water due to the presence of aquaporins. As a result, the cytosolic osmolality is always close to that of the surrounding interstitium. It is the luminal membrane and tight junctions where most of the variability lies. The segments of the renal tubule fall into several general categories with regard to water permeability: (1) Only in the proximal tubule are the tight junctions significantly permeable to water. The luminal membranes of the proximal tubule cells are also highly permeable to water. (2) The luminal membranes of the early parts of the descending thin limb of Henle's loop also have a very high water permeability. (3) The luminal membranes of the ascending limbs of Henle's loop (both thin and thick; recall from Chapter 1 that only long loops have ascending thin limbs) and the luminal membranes of the distal convoluted tubule are always relatively water impermeable, as are the tight junctions. (4) The water permeability of the luminal membranes of the collecting-duct system is intrinsically low but can be regulated so that it increases substantially. These differences in water permeability account for the sites of water reabsorption as well as the large range of water reabsorption given for the collecting-duct system in Table 6–2.

The ability of the kidneys to produce low-volume hyperosmotic urine is a major determinant of one's ability to survive without water, which for most people is several days, and even longer under optimal conditions. The human kidney can produce a maximal urinary concentration of 1400 mOsm/kg in extreme dehydration. This is almost 5 times the osmolality of plasma. The sum of the urea, sulfate, phosphate, other waste products, and a small number of nonwaste ions excreted each day normally averages approximately 600 mOsm/day. Therefore, the minimum volume of water in which this mass of solute can be dissolved is roughly 600 mmol/1400 mOsm/L = 0.43 L/day.

This volume of urine is known as the obligatory water loss. It is not a strictly fixed value but changes with different physiological states. For example, increased tissue catabolism, as during fasting or trauma, releases excess solute and so increases obligatory water loss.

The obligatory water loss contributes to dehydration when a person is deprived of water and limits survival time. For example, if we could produce urine with an osmolarity of 6000 mOsm/L, the obligatory water loss would only be 100 mL of water, and survival time would be greatly increased. A desert rodent, the kangaroo rat, does just that. This animal does not need to drink water because the water content of its food and the water produced by metabolism of the foods is sufficient to meet its needs.²

The important principles to be understood regarding individual tubular segments are (1) the mechanisms for reabsorption of sodium, chloride, and water, (2) how they relate to one another, and (3) how the amount of reabsorption quantitatively varies from one segment to another.

Proximal Tubule

As shown in Figure 6–2, sodium enters proximal tubule cells by several luminal entry steps. In the early portion, a large fraction of the tubular sodium enters the cells across the luminal membrane via antiport with protons from within the cells. Given the large amount of sodium reabsorbed compared with the vanishingly low levels of cellular protons, how can there be enough protons to supply the transporter? Furthermore, what happens to all those protons once in the lumen? These issues will be described thoroughly in Chapter 9, but for now we note that protons are generated continuously by combining carbon dioxide with water, a process that produces protons and bicarbonate. The protons exit the cell across the luminal membrane in exchange for sodium entry, while bicarbonate exits the cell across the basolateral membrane in symport with sodium. Many of the secreted protons combine with filtered bicarbonate to form carbon dioxide and water once again. Therefore, in the early proximal tubule, bicarbonate is a major anion reabsorbed with sodium, and the luminal bicarbonate concentration decreases markedly (Figure 6–3). The other secreted protons combine with other secreted bases as described below. Organic nutrients such as glucose are also absorbed with sodium, and their luminal concentrations decrease rapidly.

A major percentage of chloride reabsorption in the proximal tubule occurs via paracellular diffusion. The concentration of chloride in Bowman's capsule is, of course, essentially the same as in plasma (approximately 110 mEq/L). Along the early proximal tubule, however, the reabsorption of water, driven by the osmotic gradient created by the reabsorption of sodium plus its cotransported solutes and bicarbonate, causes the chloride concentration in the tubular lumen to increase somewhat above that in the peritubular capillaries (see Figure 6–3). Then, as the fluid flows through the middle and late proximal tubule, this concentration gradient, maintained by continued water reabsorption, provides the driving force for paracellular chloride reabsorption by diffusion.

There is also an important component of active chloride transport from lumen to cell in the later proximal tubule. As illustrated in Figure 6–2, it uses parallel Na–H and Cl-base antiporters. Chloride transport into the cell is powered by the downhill efflux of organic bases, particularly formate and oxalate. These bases are continuously generated in the cell by dissociation of their respective acids into a proton and the base. Simultaneously, the protons generated within the cell by the dissociation of the acids are actively transported into the lumen by the Na–H antiporters described earlier. In the lumen, the protons and organic bases recombine to form the acid, which is a neutral molecule. This nonpolar neutral acid then diffuses across the luminal membrane back into the cell, where the entire process is repeated. Notice that both the protons and the organic bases endlessly recycle, moving into the cells while paired as a neutral molecule and then moving out via separate transporters after the proton dissociates. The overall achievement of the parallel Na–H and Cl-base antiporters is the same as though the chloride and sodium were simply cotransported into the cell together. Importantly, the recycling of protons and base means that most of the protons are not accumulating in the lumen but are simply combining with the base and moving back into the cells. It should also be recognized that everything is ultimately dependent on the basolateral membrane Na-K-ATPases to establish the gradient for sodium that powers the luminal Na–H antiporter (Table 6–4).

Regarding water reabsorption, the proximal tubule has a very high permeability to water. This means that very small differences in osmolality (less than 1 mOsm/kg H₂O) suffice to drive the reabsorption of very large quantities of water, normally approximately 65% of the filtered water. This osmolality difference is created by the reabsorption of solute. The osmolality of the freshly filtered tubular fluid at the very beginning of the proximal tubule is, of course, essentially the same as that of plasma and interstitial fluid. Then, as solute is reabsorbed from the proximal tubule the luminal osmolality falls slightly (ie, the water concentration rises). Simultaneously, the reabsorbed solute tends to raise the interstitial fluid osmolality. However, interstitial osmolality rises only a small amount because the high perfusion through peritubular capillaries keeps the interstitial osmolality close to the plasma value. The osmotic gradient from lumen to interstitial fluid, although small, causes osmosis of water from the lumen across the plasma membranes via aquaporins and tight junctions into the interstitial fluid. The Starling forces across the peritubular capillaries in the interstitium favor reabsorption, as explained in Chapter 4, and so the water and solutes then move into the peritubular capillaries and are returned to the general circulation.

The Loop of Henle

Henle's loop as a whole reabsorbs proportionally more sodium and chloride (approximately 25% of the filtered loads) than water (10% of the filtered water). The result is that the sodium concentration in the tubular fluid is reduced to the range of ∼50 mEq/L and the fluid delivered to the distal nephron (distal tubule and beyond) is hypo-osmotic relative to plasma.

As shown in Table 6–2, the reabsorption of sodium chloride and reabsorption of water occur in different places. The descending limbs reabsorb water, but not sodium or chloride. Until just before the hairpin turn the luminal membranes express aquaporins, which allow water to move into the cells. The remaining portions of Henle's loop do not express luminal aquaporins and are water impermeable. The majority of nephrons are superficial nephrons and extend only to the border between the outer and inner medulla before turning. There are fewer long-looped nephrons with thin limbs extending past the border into the inner medulla. Because of this anatomical feature the vast majority of the water reabsorbed by the loop of Henle occurs in the outer medulla only.

In contrast to the descending limbs, the ascending limbs (both thin and thick) reabsorb sodium and chloride but not water. What are the mechanisms of sodium and chloride reabsorption by the ascending limbs? Water reabsorption in the descending limb concentrates luminal sodium and chloride, creating a favorable gradient for passive reabsorption. The thin ascending limb cells express chloride channels in both luminal and basolateral membranes through which passive chloride reabsorption occurs. The tight junctions are somewhat leaky to sodium, and so sodium follows the chloride.

As tubular fluid enters the thick ascending limb (at the junction between inner and outer medulla), the transport properties of the epithelium change again, and active processes become dominant. And because most nephrons are short-looped and do not have thin ascending limbs, the vast majority of sodium and chloride reabsorbed by the loop of Henle occurs in the thick ascending limbs in the outer medulla (and of course in the cortex, because all thick ascending limbs continue until they reach their parent Bowman's capsules).

As shown in Figure 6–4, the major luminal entry step for the sodium and chloride in thick ascending limbs is via the Na-K-2Cl symporter. This symporter is the target for a major class of diuretics collectively known as the loop diuretics, which include the drugs furosemide (Lasix) and bumetanide. The stoichiometry of the Na-K-2Cl symporter has several important consequences. First, it requires that equal amounts of potassium and sodium be transported into the cell. However, there is far less potassium in the lumen than sodium, and it seems that the lumen would be depleted of potassium long before very much sodium was reabsorbed. Interestingly, the luminal membrane has a large number of potassium channels that allow much of the potassium to leak back, that is, potassium recycles between the cytosol and lumen. Thus, under normal circumstances, luminal potassium does not limit sodium and chloride reabsorption through Na-K-2Cl symporters.

The Na-K-2Cl symporter moves twice as much chloride as sodium into the cell; therefore, twice as much chloride must also exit the cell across the basolateral membrane. The chloride leaves by a combination of chloride channels and chloride–potassium symporters, while sodium leaves primarily via the Na-K-ATPase. But, as there are 2 chloride ions leaving the cell for every 1 sodium ion, where does the rest of the sodium come from to balance this flux of chloride? It moves paracellularly. The movement of chloride sets up a lumen-positive potential. This drives luminal cations, specifically including sodium, paracellularly through the tight junctions. Thus, about half the sodium moves through the cells and half moves by paracellular diffusion. There is also some entrance of sodium via apical Na/H antiporters that are responsible for reabsorption of most of the remaining tubular bicarbonate. Of course, none of these transcellular or paracellular mechanisms would work without the continuous operation of the Na-K-ATPase in the basolateral membrane.

To summarize the most important features of the loop of Henle, the descending limb reabsorbs water but not sodium chloride, mostly in the outer medulla. The ascending limb reabsorbs sodium chloride but not water, mostly in the outer medulla and cortex. The net of the loop as a whole is reabsorption of more salt than water. The ascending limb is called a diluting segment, because the fluid leaving the loop to enter the distal convoluted tubule is hypo-osmotic (more dilute) compared with plasma.

Distal Convoluted Tubule

Tubular elements beyond the loop of Henle are collectively called the “distal nephron,” beginning with the distal convoluted tubule. The distal convoluted tubule lies entirely within the cortex, beginning just after the macula densa where the tubule passes between the afferent and efferent arterioles at the vascular pole of Bowman's capsule. It parallels the activity of the thick ascending limb by reabsorbing salt without water, and therefore is also a diluting segment, although it uses different transport mechanisms. The major luminal entry step in the active reabsorption of sodium and chloride by the distal convoluted tubule is via the Na–Cl symporter (Figure 6–5). The characteristics of this transporter differ significantly from the Na-K-2Cl symporter. It is sensitive to different drugs. In particular, the Na–Cl symporter is blocked by the thiazide diuretics, which makes it a major site for pharmacological intervention. Sodium exits the cell by the Na-K-ATPase, while chloride leaves via channels and a K–Cl symporter.

Connecting Tubule and Collecting-Duct System

Elements beyond the distal tubule begin an entirely new set of tasks. It is here that water is reabsorbed in highly variable amounts depending on body conditions. These elements continue to reabsorb additional sodium and chloride and have major mechanisms to control potassium and acid/base excretion, as described in later chapters.

Anatomically, the tubular elements beyond the distal tubule can be divided into the connecting tubule, cortical collecting duct, outer medullary collecting duct, and inner medullary collecting duct (recall that each nephron is a separate entity up to the cortical collecting duct, at which point several connecting tubules join to form one collecting duct). For the moment we will consider these elements together. The tubular epithelium is characterized by several cell types—principal cells and at least 3 types of intercalated cells. Reabsorption of sodium and water occurs in principal cells (so called because they make up approximately 70% of the cells; Figure 6–6). Principal cells also play a major role in maintaining potassium homeostasis (see Chapter 8). As the principal cells reabsorb sodium, the luminal entry step is via epithelial sodium channels (ENaC). Regulation of this entry step is enormously important for whole-body physiology, and we expand on this topic in Chapter 7.

The handling of chloride in the distal nephron is rather complex because the handling of solutes other than sodium makes up a significant component of ion transport. Chloride moves through several types of transporters in the intercalated cells that are key players in potassium and acid/base transport described in Chapter 9 (see Figure 9–3). As in all nephron segments, net anion flux must equal net cation flux. Thus chloride flux must match the net of sodium, potassium, and acid-base transport.

How does the collecting duct system handle water? As tubular fluid leaves the diluting segments and enters the collecting duct system the luminal osmolality is low, typically a little above 100 mOsm/kg. Therefore, there is an osmotic gradient favoring water reabsorption. Water permeability in the principal cells of the collecting duct system is finely regulated by circulating antidiuretic hormone (ADH; see Figure 6–6). The inner medullary collecting duct has at least a finite water permeability even in the absence of ADH, but the outer medullary and cortical regions have a vanishingly low water permeability without the actions of this hormone.

Depending on levels of ADH, water permeability for most of the collecting-duct system can be very low, very high, or everything in between. When water permeability is very low (no ADH), the hypo-osmotic fluid entering the collecting-duct system from the distal convoluted tubule remains hypo-osmotic as it flows along the ducts. When this fluid reaches the medullary portion of the collecting ducts, there is now a very large osmotic gradient favoring reabsorption. Some water is reabsorbed in the medullary region, but most of the water flows on to the ureter. The result is the excretion of a large volume of very hypo-osmotic (dilute) urine, or water diuresis. Recall that almost 25% of the filtered water is still within the tubule at the beginning of the collecting duct system, so this amounts to a huge amount of nonreabsorbed water.

Even when very little water reabsorption occurs beyond the loop of Henle, the reabsorption of sodium is not reduced to any great extent. Therefore, the tubular sodium concentration can be lowered almost to zero in these segments, and the osmolality can approach 50 mOsm/kg, most of the osmotic content being made up of urea and other organic waste. The low sodium is possible because these tubular segments are “tight” epithelia, and there is very little back-leak of sodium from interstitium to tubular lumen despite the large electrochemical gradient favoring diffusion.

What happens when the collecting-duct system's water permeability is very high (high ADH)? As the hypo-osmotic fluid enters the cortical segments of the collecting-duct system most of the water is rapidly reabsorbed. This is driven by the large difference in osmolality between the hypo-osmotic luminal fluid and the iso-osmotic (285 mOsm/kg) interstitial fluid of the cortex. In essence, the cortical collecting duct is reabsorbing the large volume of water that did not accompany solute reabsorption in the ascending limbs of Henle's loop and distal convoluted tubule. In other words, the cortical collecting duct reverses the dilution carried out by the diluting segments. Once the osmolality of the luminal fluid approaches that of the interstitial fluid, the cortical collecting duct then behaves analogously to the proximal tubule, reabsorbing approximately equal proportions of solute (mainly sodium chloride) and water. The result is that the tubular fluid leaving the cortical collecting duct to enter the medullary collecting duct is greatly reduced in volume and is iso-osmotic compared with cortical plasma.

In the medullary collecting duct, solute reabsorption continues but water reabsorption is proportionally even greater. The tubular fluid becomes very hyperosmotic and even further reduced in volume in its passage through the medullary collecting ducts because the interstitial fluid of the medulla is very hyperosmotic (for reasons discussed later).

How does ADH convert epithelial water permeability from very low to very high? An alternative name for ADH is vasopressin, because the hormone can constrict arterioles and thus increase arterial blood pressure, but ADH's major renal effect is antidiuresis (ie, “against a high urine volume”). ADH acts in the collecting ducts on the principal cells, the same cells that reabsorb sodium. The renal receptors for ADH (vasopressin type 2 receptors) are in the basolateral membrane of the principal cells and are different from the vascular receptors (vasopressin type 1). The binding of ADH by its receptors results in the activation of adenylate cyclase, which catalyzes the intracellular production of cyclic adenosine monophosphate (cAMP). This second messenger then induces, by a sequence of events, the migration of intracellular vesicles to, and their fusion with, the luminal membrane. Recall from Chapter 4, this is one of ways of regulating membrane permeability. The vesicles contain aquaporin 2, through which water can move, so the luminal membrane becomes highly permeable to water. In the absence of ADH, the aquaporins are withdrawn from the luminal membrane by endocytosis. (As stated earlier, the water permeability of the basolateral membranes of renal epithelial cells is always high because of the constitutive presence of other aquaporin isoforms; thus, the permeability of the luminal membrane is rate limiting.)

We have said that the kidneys can produce a range of urine osmolalities depending on the levels of ADH. The production of hypo-osmotic urine is, we hope by now, an understandable process: The tubules (particularly the thick ascending limb of Henle's loop) reabsorb relatively more solute than water, and the dilute fluid that remains in the lumen is excreted. The production of hyperosmotic urine is also straightforward in that reabsorption of water from the lumen into a hyperosmotic interstitium concentrates that luminal fluid, leaving concentrated urine to be excreted. The question is, how do the kidneys generate a medullary interstitium that is hyperosmotic relative to plasma? Not only is the medullary interstitium hyperosmotic, but there is a gradient of osmolality, increasing from a nearly iso-osmotic value at the corticomedullary border, to a maximum of greater than 1000 mOsm/kg at the papilla. This peak value is not rigidly fixed; it is a variable that changes depending on conditions. It is highest during periods of water deprivation and dehydration, when urinary excretion is lowest, and is “washed out” to only approximately half of that during excess hydration and urinary excretion is high. Some aspects of how the kidneys generate a medullary osmotic gradient are still uncertain. However, the essential points are clear, and it is these essential points on which we now focus.³

We should first differentiate between the development of the medullary osmotic gradient, as opposed to its maintenance once established. In the steady state there must be mass balance, that is, every substance that enters the medulla via tubule or blood vessel must leave the medulla via tubule or blood vessel. However, during development of the gradient there are transient accumulations of solute, and during washout of the gradient there are losses. In describing the medullary osmotic gradient, it is easiest conceptually to start from a condition in which there is no gradient, and then follow its development over time. The main components of the system that develops the medullary osmotic gradient are (1) active NaCl transport by the thick ascending limb, (2) the parallel arrangement of blood vessels and tubular segments in the medulla, with descending components in close apposition to ascending components, and (3) the recycling of urea between the medullary collecting ducts and the deep portions of the loops of Henle (Figure 5–4).

To develop the osmotic gradient in the medullary interstitium, there must be deposition of solute in excess of water. It is reabsorption of sodium and chloride by the thick ascending limb in excess of water reabsorbed in the thin descending limbs that accomplishes this task. At the junction between the inner and outer medulla, the ascending limbs of all loops of Henle, whether long or short, turn into thick regions and remain thick all the way back until they reach the original Bowman's capsules from which they arose in the cortex. As they reabsorb solute without water and dilute the luminal fluid, they simultaneously add solute without water to the surrounding interstitium. This action of the thick ascending limb is absolutely essential and is the key to everything else that happens. If transport in the thick ascending limb is inhibited (by loop diuretics that block the Na-K-2Cl symporter), the lumen is not diluted and the interstitium is not concentrated, and the urine becomes iso-osmotic. For those portions of the thick ascending limb in the cortex, the reabsorbed solute simply mixes with material reabsorbed by the nearby proximal convoluted tubules. Because the cortex contains abundant peritubular capillaries and a high blood flow, the reabsorbed material immediately moves into the vasculature and returns to the general circulation. However, in the medulla, the vascular anatomy is arranged differently and total blood flow is much lower. Solute that is reabsorbed and deposited in the outer medullary interstitium during the establishment of the osmotic gradient is not immediately removed, that is, it accumulates. The degree of accumulation before a steady state is reached is a function of the arrangement of the vasa recta, their permeability properties and the volume of blood flowing within them.

Imagine first a hypothetical situation of no blood flow. Sodium would accumulate in the outer medulla without limit, because there would be no way to remove it. But, of course the outer medulla is perfused with blood, as are all tissues. Blood enters and leaves the outer medulla through parallel bundles of descending and ascending vasa recta. These vessels are permeable to sodium. Therefore sodium enters the vasa recta driven by the rise in concentration in the surrounding interstitium. Sodium entering the ascending vessels returns to the general circulation, but sodium in the descending vessels is distributed deeper into the medulla, where it diffuses out across the endothelia of the vasa recta and the interbundle capillaries that they feed, thereby raising the sodium content (and osmolality) throughout the medulla. It is here that the anatomy of the vasculature becomes particularly important. If medullary blood with its somewhat elevated sodium concentration simply flowed into a venous drainage system, there would be no additional increase in sodium concentration. However, the interbundle capillaries drain into ascending vasa recta that lie near descending vasa recta. The walls of the ascending vasa recta are fenestrated, allowing movement of water and small solutes between plasma and interstitium. As the sodium concentration of the medullary interstitium rises, blood in the ascending vessels also takes on an increasingly higher sodium concentration. However, blood entering the medulla always has a normal sodium concentration (approximately 140 mEq/L). Accordingly, some of the sodium begins to re-circulate, diffusing out of ascending vessels and reentering nearby descending vessels that contain less sodium. The process of moving from ascending to descending vessels is called countercurrent exchange. At this point sodium is entering descending vasa recta from 2 sources—re-circulated sodium from the ascending vasa recta, and new sodium from the thick ascending limbs. Over time everything reaches a steady state in which the amount of new sodium entering the interstitium from thick ascending limbs matches the amount of sodium leaving the interstitium in ascending vasa recta. At its peak, the concentration of sodium in the medulla may reach 300 mEq/L, more than double its value in the general circulation. Since sodium is accompanied by an anion, mostly chloride, the contribution of salt to the medullary osmolality is approximately 600 mOsm/kg.

What happens to water in the medulla during this time? Before answering this question, several constraining principles should be kept in mind: (1) While solute can accumulate without a major effect on renal volume, the amount of water in the medullary interstitium must remain nearly constant; otherwise the medulla would undergo significant swelling or shrinking. (2) Because water is always being reabsorbed from the medullary tubules into the interstitium (from descending thin limbs and medullary collecting ducts), that water movement must be matched by equal water movement from the interstitium to the vasculature. (3) Blood entering the medulla has passed through glomeruli, thereby concentrating the plasma proteins (and has not been subject to dilution by water uptake in peritubular capillaries). While the overall osmotic content (osmolality) of this blood is essentially isosmotic with systemic plasma, its oncotic pressure is considerably higher.

The challenge for the kidneys is to prevent dilution of the hyperosmotic interstitium by water reabsorbed from the tubules and by water diffusing out of the iso-osmotic blood entering the medulla. The endothelial cells of descending vasa recta contain aquaporins. Water is drawn osmotically into the outer medullary interstitium by the high salt content in a manner similar to water being drawn out of tubular elements. At first glance it seems that this allows the undesired diluting effect to actually take place. But, of course, solute is also constantly being added from the nearby thick ascending limbs. The loss of water from descending vasa recta in the outer medulla serves the useful purpose of raising the osmolality of blood penetrating the inner medulla and decreasing its volume, thereby reducing the tendency to dilute the inner medullary interstitium. The ascending vasa recta have a fenestrated endothelium, allowing free movement of water and small solutes. Since the oncotic pressure is high, water entering the interstitium of the outer medulla from descending vasa recta is taken up by ascending vasa recta and removed from the medulla. Just as there is countercurrent exchange of solute between descending and ascending vessels, there is countercurrent exchange of water. In descending vessels water leaves and solute enters, while in ascending vessels water enters and solute leaves (see Figures 1–5 and 6–7). It is the countercurrent movement of water that prevents entering blood from diluting the inner medulla. And of course, all water reabsorbed from tubular elements is also taken up by ascending vasa recta and removed, thereby preserving constancy of total medullary water content.

The magnitude of blood flow in the vasa recta is a crucial variable. The peak osmolality in the interstitium depends on the ratio of sodium pumping by the thick ascending limbs to blood flow in the vasa recta. If this ratio is high (meaning low blood flow), water from the isosmotic plasma entering the medulla in descending vasa recta does not dilute the hyperosmotic interstitium. In effect the “salt wins” and osmolality remains at a maximum. But in conditions of water excess, this ratio is very low (high blood flow) and the diluting effect of water diffusing out of descending vasa recta is considerable. In part the tendency to dilute is controlled by ADH. As well as increasing water permeability in the collecting ducts, ADH vasoconstricts the pericytes that surround descending vasa recta, thereby limiting their blood flow. Thus when water is being conserved (high ADH), medullary blood flow is low and the high osmolality is preserved.

The third major player involved in the development of the medullary osmotic gradient is urea. As indicated above, the peak osmolality in the renal papilla reaches over 1000 mOsm/kg. Approximately half of this is accounted for by sodium and chloride, and most of the rest is (500–600 mOsm/kg) is accounted for by urea. To develop and maintain such a high concentration of urea (remember that the normal plasma concentration is only approximately 5 mmol/L) there must be a process of recycling. This involves the tubules as well as the vasa recta.

We described this recycling process in Chapter 5 and review it here. Urea is freely filtered and approximately half is reabsorbed in the proximal tubule. Urea is secreted in the loop of Henle (thin regions), driven by the high urea concentration in the medullary interstitium. This essentially restores the amount of tubular urea back to the filtered load. From the end of the thin limbs to the inner medullary collecting ducts, little urea transport occurs; so all urea arriving at the thick ascending limb is still there at the start of the inner medullary collecting ducts. Because the vast majority of water has already been reabsorbed prior to the inner medullary collecting ducts (by the cortical and outer medullary collecting ducts), the luminal urea concentration has risen up to 50 times its plasma value (ie, 500 mmol/L or more). In the inner medullary collecting ducts, approximately half the urea is reabsorbed via urea uniporters and the other half is excreted. Because blood flow in this region is so low in conditions of antidiuresis (high ADH), the reabsorbed urea accumulates and raises the interstitial concentration close to that in the lumen. (It is this high interstitial concentration that drives secretion in the thin limbs.) The combination of high urea, along with the high sodium and chloride, brings the medullary osmolality to a value exceeding 1000 mOsm/kg. The importance of urea in contributing to the medullary osmotic gradient is emphasized in the case of low protein intake, which results in a greatly reduced metabolic production of urea. In this condition, the ability of the kidneys to generate a hyperosmotic medullary interstitium is reduced.

Review of Control by ADH

As emphasized earlier, a key regulator of the osmotic gradient is ADH, which in addition to raising water permeability in the cortical and medullary collecting ducts and constricting pericytes surrounding descending vasa recta, also raises urea permeability in the inner medullary collecting ducts by stimulating a specific ADH-sensitive isoform of the urea uniporters. Consider how this affects the medullary osmotic gradient. When a person is dehydrated, glomerular filtration rate (GFR) is somewhat low and levels of ADH are high. The extraction of water in the cortical collecting duct removes most of the water from the lumen (and makes it iso-osmotic with the cortical interstitium, ie, approximately 300 mOsm/kg). Then, as the remaining but greatly reduced volume flows through the high osmolality medulla, further concentration occurs. The increased urea permeability signaled by ADH greatly assists in generating the medullary osmotic gradient by permitting the recycling of urea.

Contrast this with a state of overhydration. Some of the medullary solute is washed out and the magnitude of the osmotic gradient is reduced. How does this occur? In states of overhydration levels of ADH are low. GFR is substantial. Most of the tubular fluid entering the cortical collecting ducts flows on through to the medullary collecting ducts. Therefore tubular urea does not become concentrated nearly as much. A high volume of very dilute fluid with a modest urea concentration is delivered to the inner medullary collecting ducts. In contrast to the cortical and outer medullary collecting ducts, which are nearly water impermeable in the absence of ADH, the inner medullary collecting ducts have finite water permeability in the absence of ADH. Although this water permeability is not large, the osmotic driving force is huge, so substantial amounts of water are reabsorbed. (However, even more is not reabsorbed, and so the urine volume is still very large.) Not much urea is reabsorbed; in fact, it may be secreted briefly because the luminal urea concentration is lower than in the medullary interstitium. The result of the water reabsorption and the low urea reabsorption is that the inner medulla becomes partially diluted (ie, the urea concentration and total osmolality of the medullary interstitium decrease over time). The osmolality falls to approximately half of its value, from well over 1000 mOsm/kg down to 500 to 600 mOsm/kg, (Table 6–5). Figure 6–7 depicts renal water fluxes in the 2 extremes of maximum diuresis and antidiuresis.

To summarize the generation of the renal osmotic gradient (Figure 6–8), salt (without water) is deposited in the interstitium by the thick ascending limb. That salt accumulates because of a combination of low blood flow and countercurrent exchange between ascending and descending vasa recta. Some of the salt is distributed deeper into the medulla by the vasa recta. Adding to the osmolality of the inner medulla is urea, which recycles from the inner medullary collecting ducts to the thin limbs of the loop of Henle. Countercurrent movement of water from descending to ascending vasa recta limits the tendency of entering blood to dilute of the medullary interstitium.

We conclude this chapter by addressing 2 frequently asked questions. First, even if the blood does not dilute the interstitium, why doesn't water reabsorbed from the collecting ducts under conditions of high ADH dilute the interstitium and abolish the osmotic gradient? The simple answer is that more solute is deposited into the medulla than water. While some water is reabsorbed from medullary collecting ducts aided by the actions of ADH, most of the water has already been reabsorbed by the cortical collecting ducts, so that the amount remaining to be reabsorbed, and potentially dilute the interstitium, is relatively small. The competing tendencies to dilute the interstitium with water and to concentrate the interstitium with salt reach a steady state in which osmolality is high. It is this balance that sets the upper limit on medullary osmolality.

The other question that often arises concerns medullary water reabsorption during diuresis when ADH is low. How can more water be reabsorbed in the medulla without the actions of ADH than during antidiuresis when ADH is high and the body is conserving water? This seeming paradox is resolved by realizing that during diuresis the fluid entering the medullary collecting ducts is very dilute, and so provides a huge driving force for reabsorption. Furthermore, the inner medullary collecting duct has a low, but finite water permeability even without ADH. This combination drives a moderate amount of water reabsorption. However, this amount is greatly exceeded by the amount not reabsorbed, that is, excreted.

Figure 6–9 summarizes the previously described changes in volume and osmolality of the tubular fluid as it flows along the nephron and emphasizes how, once fluid enters the collecting-duct system, the osmolality depends very much on levels of ADH.

• Most of the body consists of fluid compartments divided into the intracellular fluid (ICF, all the cytosolic volumes collectively) and the extracellular fluid (ECF), consisting mostly of interstitial fluid and blood plasma.

• The reabsorption of most of the filtered water, anions (primarily chloride and bicarbonate), and osmotic content is linked directly to the active reabsorption of sodium.

• In all conditions, the vast majority (roughly two thirds) of the sodium, chloride, bicarbonate, and filtered volume is reabsorbed iso-osmotically in the proximal tubule.

• The loop of Henle reabsorbs some water and proportionally even more sodium, thereby diluting the tubular fluid.

• The distal tubule continues the reabsorption of sodium without water and, along with the loop of Henle, is considered a “diluting segment.”

• Reabsorption of water in the distal nephron (connecting tubule and collecting ducts) is highly variable depending on hydration status, allowing the kidneys to excrete large amounts of water or to conserve almost all of it.

• Levels of ADH determine whether the hypo-osmotic fluid entering the distal nephron is excreted largely as is or subsequently reabsorbed.

• The conservation of water and concentration of the urine requires the reabsorption of water into the high osmolality medullary interstitium.

• The medullary osmotic gradient is created by (1) transport of salt without water into the medullary interstitium by the thick ascending limb, (2) low-volume countercurrent blood flow in the vasa recta, and (3) recycling of urea.