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.
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.
Major pathways for reabsorption of sodium, chloride, and water in the proximal tubule. The entire proximal tubule is the major site for reabsorption of salt and water. Sodium entry is coupled to the secretion or uptake of a variety of substances, the major one being secreted hydrogen ions via the NHE-3 antiporter. These hydrogen ions combine with filtered bicarbonate and secreted organic base (see text and Chapter 9 for further explanation). Additional sodium enters in symport with glucose, amino acids, and phosphate. Sodium is transported to the interstitium mostly via the basolateral Na-K-ATPase, but also in symport with bicarbonate. (The coupling between sodium and bicarbonate is described fully in Chapter 9.) Chloride that enters in antiport with organic base leaves mostly via channels. In addition, a substantial amount of chloride is reabsorbed paracellularly. Water moves both paracellularly and intracellularly via aquaporins. ATP, adenosine triphosphate.
Concentrations of solutes in tubular fluid relative to concentrations in plasma as a function of distance along proximal tubule. Solute concentrations change due to water reabsorption and solute transport. Inulin is not transported, but its concentration rises due to the reabsorption of water. Sodium concentration does not change because sodium and water are reabsorbed in equal proportions. The concentrations of the other substances fall because they are reabsorbed in progressively greater proportions than water. The value for glucose falls to zero because virtually all is reabsorbed. (Based on Rector FC. Am J Physiol. 1983;249:F461)
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).
Table 6–4.Summary of mechanisms by which reabsorption of sodium drives reabsorption of other substances in the proximal tubule ||Download (.pdf) Table 6–4. Summary of mechanisms by which reabsorption of sodium drives reabsorption of other substances in the proximal tubule
Creates transtubular osmolality difference, which favors reabsorption of water by osmosis; in turn, water reabsorption concentrates many luminal solutes (eg, chloride and urea), thereby favoring their reabsorption by diffusion.
Achieves reabsorption of many organic nutrients, phosphate, and sulfate by symport across the luminal membrane.
Achieves reabsorption of bicarbonate by secretion of hydrogen ions by antiport across the luminal membrane. These hydrogen ions convert filtered bicarbonate to CO2 and water, while bicarbonate generated within the cell is transported into the interstitium as described in Chapter 9.
Achieves reabsorption of chloride by parallel Na/H and Cl/base antiporters.
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.
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).
The Na-K-2Cl symporter in the thick ascending limb is the machine that separates salt from water
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.
Major transport pathways for sodium and chloride in thick ascending limb of the loop of Henle. The key transporter in the thick ascending limb is the Na-K-2Cl symporter (NKCC), which is the target for inhibition by loop diuretics like furosemide and bumetanide. The cells contain potassium channels that recycle potassium from the cell interior to the lumen and to the interstitium (see Chapter 8). Besides transcellular routes, considerable sodium moves paracellularly in response to the lumen positive potential. The apical membranes and tight junctions have very low water permeability. Because the cells reabsorb salt, but not water, the thick ascending limb is the region of the nephron in which salt is separated from water. This ultimately allows water excretion and salt excretion to be controlled independently. Defects in NKCC, the recycling potassium channel, and the basolateral chloride channel lead, respectively, to the 3 different types of Bartter syndrome. ATP, adenosine triphosphate.
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.
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.
Most filtered water is reabsorbed proximally. Variable amounts of what remains are reabsorbed distally under the control of ADH.
Major transport pathways for sodium and chloride in the distal convoluted tubule. The apical membrane contains the Na-Cl symporter (NCC), which is the target for inhibition by thiazide diuretics. There is also some sodium reabsorption via apical sodium channels (ENaCs). The apical membranes and tight junctions have very low water permeability. A defect in NCC leads to Gitelman syndrome. ATP, adenosine triphosphate.
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.
Major transport pathways for sodium, chloride, and water in principal cells of the cortical collecting duct, which are the major cell type. Sodium reabsorption is via apical sodium channels (ENaC). Chloride reabsorption is mainly transcellular via intercalated cells (see Chapter 9). Water reabsorption is via aquaporins, the activity of which is controlled by the antidiuretic hormone (ADH). ATP, adenosine triphosphate.
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.)