Transport of Acids and Bases

Whereas the lungs excrete the large amount of CO ₂ formed by metabolism, the kidneys are crucial for excreting nonvolatile acids

The kidneys play a critical role in helping the body rid itself of excess acid that accompanies the intake of food or that forms in certain metabolic reactions. By far, the largest potential source of acid is CO ₂ production ( Table 39-1 , section A), which occurs during oxidation of carbohydrates, fats, and most amino acids (see pp. 1185–1187 ). An adult ingesting a typical Western diet produces ~15,000 mmol/day of CO ₂ . This CO ₂ would act as an acid if it went on to form H ⁺ and (see p. 630 ). Fortunately, the lungs excrete this prodigious amount of CO ₂ by diffusion across the alveolar-capillary barrier (see p. 673 ), preventing the CO ₂ from forming H ⁺ .

However, metabolism also generates nonvolatile acids —such as sulfuric acid, phosphoric acid, and various organic acids—that the lungs cannot handle (see Table 39-1 , section B). In addition, metabolism generates nonvolatile bases, which end up as (see Table 39-1 , section C). Sub­tracting the metabolically generated base from the meta­bolically generated acid leaves a net endogenous H ⁺ production of ~40 mmol/day for a person weighing 70 kg. The strong acids contained in a typical Western acid-ash diet (20 mmol/day of H ⁺ gained) and the obligatory loss of bases in stool (10 mmol/day of OH ⁻ lost) represent an additional acid load to the body of 30 mmol/day. Thus, the body is faced with a total load of nonvolatile acids (i.e., not CO ₂ ) of ~70 mmol/day—or ~1 mmol/kg body weight—derived from metabolism, diet, and intestinal losses. The kidneys handle this acid load by “dividing” 70 mmol/day of carbonic acid (H ₂ CO ₃ ): excreting ~70 mmol/day of H ⁺ into the urine and simultaneously transporting 70 mmol/day of new into the blood. Once in the blood, this new neutralizes the daily load of 70 mmol of nonvolatile acid.

Were it not for the tightly controlled excretion of H ⁺ by the kidney, the daily load of ~70 mmol of nonvolatile acids would progressively lower plasma pH and, in the process, exhaust the body's stores of bases, especially . The result would be death by relentless acidification. Indeed, one of the characteristic symptoms of renal failure is severe acidosis caused by acid retention. N39-1 The kidneys continuously monitor the acid-base parameters of the extracellular fluid (ECF) and adjust their rate of acid secretion to maintain the pH of ECF within narrow limits.

In summary, although the lungs excrete an extremely large amount of a potential acid in the form of CO ₂ , the kidneys play an equally essential role in the defense of the normal acid-base equilibrium, because they are the sole effective route for neutralizing nonvolatile acids.

To maintain acid-base balance, the kidney must not only reabsorb virtually all filtered but also secrete generated nonvolatile acids

In terms of acid-base balance, the major task of the kidney is to secrete acid into the urine and thus to neutralize the nonvolatile acids that metabolism produces. However, before the kidney can begin to achieve this goal, it must deal with a related and even more serious problem: retrieving from the tubule fluid virtually all filtered by the glomeruli.

Each day, the glomeruli filter 180 L of blood plasma, each liter containing 24 mmol of , so that the daily filtered load of is 180 L × 24 mM = 4320 mmol. If this filtered were all left behind in the urine, the result would be equivalent to an acid load in the blood of 4320 mmol, or a catastrophic metabolic acidosis (see p. 635 ). The kidneys avoid this problem by reclaiming virtually all the filtered through secretion of H ⁺ into the tubule lumen and titration of the 4320 mmol/day of filtered to CO ₂ and H ₂ O.

After the kidney reclaims virtually all the filtered (i.e., 4320 mmol/day), how does it deal with the acid load of 70 mmol/day produced by metabolism, diet, and intestinal losses? If we simply poured 70 mmol of nonvolatile acid into the ~1.5 L of “unbuffered” urine produced each day, urinary [H ⁺ ] would be 0.070 mol/1.5 L = 0.047 M, which would correspond to a pH of ~1.3. The lowest urine pH that the kidney can achieve is ~4.4, which corresponds to an [H ⁺ ] that is three orders of magnitude lower than required to excrete the 70 mmol/day of nonvolatile acids. The kidneys solve this problem by binding the H ⁺ to buffers that the kidney can excrete within the physiological range of urinary pH values. Some of these buffers the kidney filters —for example, phosphate, creatinine, and urate. Because of its favorable p K of 6.8 and its relatively high rate of excretion, phosphate is the most important nonvolatile filtered buffer. The other major urinary buffer is , which the kidney synthesizes. After diffusing into the tubule lumen, the NH ₃ reacts with secreted H ⁺ to form . Through adaptive increases in the synthesis of NH ₃ and excretion of , the kidneys can respond to the body's need to excrete increased loads of H ⁺ .

The kidney does not simply eliminate the 70 mmol/day of nonvolatile acids by filtering and then excreting them in the urine. Rather, the body deals with the 70-mmol/day acid challenge in three steps:

• Step 1: Extracellular neutralizes most of the H ⁺ load:

  

• Thus, decreases by an amount that is equal to the H ⁺ it consumes, and an equal amount of CO ₂ is produced in the process. buffers (see p. 635 ) in the blood neutralize most of the remaining H ⁺ load:

  

• Thus, B ⁻ , too, decreases by an amount that is equal to the H ⁺ it consumes. A very tiny fraction of the H ⁺ load (<0.001%; see Fig. 28-7, panel 2A ) escapes buffering by either or B ⁻ . This remnant H ⁺ is responsible for a small drop in the extracellular pH.

• Step 2: The lungs excrete the CO ₂ formed by the process in Equation 39-1 . The body does not excrete the BH generated by the process in Equation 39-2 , but rather converts it back into B ⁻ , as discussed below.

• Step 3: The kidneys regenerate the and B ⁻ in the ECF by creating new at a rate that is equal to the rate of H ⁺ production (i.e., ~70 mmol/day). Thus, over the course of a day, 70 mmol more exits the kidneys via the renal veins than entered via the renal arteries. Most of this new replenishes the consumed by the neutralization of nonvolatile acids, so that extracellular [ ] is maintained at ~24 mM. The remainder of this new regenerates B ⁻ :

  

• Again, the lungs excrete the CO ₂ formed as indicated in Equation 39-3 , just as they excrete the CO ₂ formed by the process in Equation 39-1 . Thus, by generating new , the kidneys maintain constant levels of both and the deprotonated forms of buffers (B ⁻ ) in the ECF.

Table 39-2 lists the three components of net urinary acid excretion. Historically, component 1 is referred to as titratable acid, the amount of base one must add to a sample of urine to bring its pH back up to the pH of blood plasma. The titratable acid does not include the H ⁺ the kidneys excrete as , which is component 2. Because the p K of the equilibrium exceeds 9, almost all of the total ammonium buffer in the urine is in the form of , and titrating urine from an acid pH to a pH of 7.4 will not appreciably convert to NH ₃ . If no filtered were lost in component 3, the generation of new by the kidneys would be the sum of components 1 and 2. To the extent that filtered is lost in the urine, the new must exceed the sum of components 1 and 2. N39-2

TABLE 39-2

Components of Net Urinary Acid Excretion

		1		2		3
Net urinary acid excretion	=	Excreted H + bound to phosphate (as ), creatinine, and uric acid	+	Excreted H + bound to NH 3 (as )	–	Excretion of filtered

The kidneys also can control and B ⁻ following an alkaline challenge, produced, for example, by ingesting alkali or by vomiting (which leads to a loss of HCl, equivalent to a gain in NaHCO ₃ ). The kidney responds by decreasing net acid excretion—that is, by sharply reducing the excretion rates of titratable acid and . The result is a decrease in the production of new . With an extreme alkali challenge, the excretion of urinary also increases and may exceed the combined rates of titratable acid and excretion. In other words, component 3 in Table 39-2 exceeds the sum of components 1 and 2, so that “net acid excretion” becomes negative, and the kidney becomes a net excretor of alkali. In this case, the kidneys return less to the ECF via the renal veins than entered the kidneys via the renal arteries.

Secreted H ⁺ titrates to CO ₂ ( reabsorption) and also titrates filtered buffers and endogenously produced NH ₃

As we have seen, the kidney can reabsorb nearly all of the filtered and excrete additional acid into the urine as both titratable acid and . The common theme of these three processes is H ⁺ secretion from the blood into the lumen. Thus, the secreted H ⁺ can have three fates. It can titrate (1) filtered , (2) filtered phosphate (or other filtered buffers that contribute to the “titratable acid”), and (3) NH ₃ , both secreted and, to a lesser extent, filtered.

Titration of Filtered (“ Reabsorption”)

Exten­sive reabsorption reclaims almost all of the filtered (>99.9%). As discussed beginning on [CR] , the kidney reabsorbs at specialized sites along the nephron. However, regardless of the site, the basic mechanism of reabsorption is the same ( Fig. 39-2 A ): H ⁺ transported into the lumen by the tubule cell titrates filtered to CO ₂ plus H ₂ O. One way that this titration can occur is by H ⁺ interacting with to form H ₂ CO ₃ , which in turn dissociates to yield H ₂ O and CO ₂ . However, the reaction H ₂ CO ₃ → H ₂ O + CO ₂ is far too slow to convert the entire filtered load of to CO ₂ plus H ₂ O. The enzyme carbonic anhydrase (CA) N18-3 —which is present in many tubule segments—bypasses this slow reaction by splitting into CO ₂ and OH ⁻ (see Table 39-1 ). The secreted H ⁺ neutralizes this OH ⁻ so that the net effect is to accelerate the production of H ₂ O and CO ₂ .

The apical membranes of these H ⁺ -secreting tubules are highly permeable to CO ₂ , so that the CO ₂ produced in the lumen, as well as the H ₂ O, diffuses into the tubule cell. Inside the tubule cell, the CO ₂ and H ₂ O regenerate intra­cellular H ⁺ and with the aid of CA. Finally, the cell exports these two products, thereby moving the H ⁺ out across the apical membrane into the tubule lumen and the out across the basolateral membrane into the blood. Thus, for each H ⁺ secreted into the lumen, one disappears from the lumen, and one appears in the blood. However, the that disappears from the lumen and the that appears in the blood are not the same molecule! To secrete H ⁺ and yet keep intracellular pH within narrow physiological limits (see pp. 644–645 ), the cell closely coordinates the apical secretion of H ⁺ and the basolateral exit of .

Two points are worth re-emphasizing. First, reabsorption does not represent net H ⁺ excretion into the urine. It merely prevents the loss of the filtered alkali. Second, even though reabsorption is simply a reclamation effort, this process consumes by far the largest fraction of the H ⁺ secreted into the tubule lumen. For example, reclaiming the 4320 mmol of filtered each day requires 4320 mmol of H ⁺ secretion, far more than the additional 70 mmol/day of H ⁺ secretion necessary for neutralizing nonvolatile acids.

Titration of Filtered Buffers (Titratable-Acid For­mation)

The H ⁺ secreted into the tubules can interact with buffers other than and NH ₃ . The titration of the non-NH ₃ , buffers (B ⁻ )—mainly , creatinine, and urate—to their conjugate weak acids (HB) constitutes the titratable acid discussed on page 823 .

The major proton acceptor in this category of buffers excreted in the urine is
, although creatinine also makes an important contribution; urate and other buffers contribute to a lesser extent. Figure 39-2 B shows the fate of H ⁺ as it protonates phosphate from its divalent form (
) to its monovalent form (
). Because low luminal pH inhibits the apical Na/phosphate cotransporter (NaPi) in the proximal tubule, and NaPi carries
less effectively than
(see pp. 785–786 ), the kidneys tend to excrete H ⁺ -bound phosphate in the urine. For each H ⁺ it transfers to the lumen to titrate
, the tubule cell generates one new
and transfers it to the blood (see Fig. 39-2 B ).

How much does the “titratable acid” contribute to net acid excretion? The following three factors determine the rate at which these buffers act as vehicles for excreting acid:

• 1

  The amount of the buffer in the glomerular filtrate and final urine. The filtered load (see p. 732 ) of , for example, is the product of plasma [ ] and glomerular filtration rate (GFR). Plasma phosphate levels may range from 0.8 to 1.5 mM (see p. 1054 ). Therefore, increasing plasma [ ] allows the kidneys to excrete more H ⁺ in the urine as . Conversely, decreasing the GFR (as in chronic renal failure) reduces the amount of available for buffering, lowers the excretion of titratable acid, and thus contributes to metabolic acidosis. Ultimately, the key parameter is the amount of buffer excreted in the urine. In the case of phosphate, the fraction of the filtered load that the kidney excretes increases markedly as plasma [phosphate] exceeds the maximum saturation (T _m ; see p. 786 ). For a plasma [phosphate] of 1.3 mM, the kidneys reabsorb ~90%, and ~30 mmol/day appear in the urine.

• 2

  The p K of the buffer. To be most effective at accepting H ⁺ , the buffer (e.g., phosphate, creatinine, urate) should have a p K value that is between the pH of the glomerular filtrate and the pH of the final urine. For example, if blood plasma has a pH of 7.4, then only ~20% of its phosphate (p K = 6.8) will be in the form of ( Table 39-3 ). Even if the final urine were only mildly acidic, with a pH of 6.2, ~80% of the phosphate in the urine would be in the form of . In other words, the kidney would have titrated ~60% of the filtered phosphate from to . Because creatinine has a p K of 5.0, lowering the pH of the tubule fluid from 7.4 to 6.2 increases the fractional protonation of creatinine from ~0.4% to only ~6%. However, urate has a p K of 5.8, so lowering pH from 7.4 to 6.2 would increase its fractional protonation from 2.5% to 28.5%.

  TABLE 39-3

  Titration of Buffers

  	% PROTONATED BUFFER
  pH	PHOSPHATE (p K = 6.8)	URATE (p K = 5.8)	CREATININE (p K = 5.0)
  7.4	20.1	2.5	0.4
  6.2	79.9	28.5	5.9
  4.4	99.6	96.2	79.9

• 3

  The pH of the urine. Regardless of the p K of the buffer, the lower is the urinary pH, the more protonated is the buffer and the greater is the amount of acid excreted with this buffer. As discussed, lowering the pH of the tubule fluid from 7.4 to 6.2 increases the protonation of creatinine from 0.4% to only ~6%. However, if the pH of the final urine is 4.4, the fractional protonation of creatinine increases to ~80% (see Table 39-3 ). Thus, creatinine becomes a much more effective buffer during acidosis, when the kidney maximally acidifies the urine.

Titration of Filtered and Secreted NH ₃ (Ammonium Excre­tion)

The third class of acceptors of luminal H ⁺ is NH ₃ . However, unlike either or the bases that give rise to “titratable acid” (e.g., ), glomerular filtration contributes only a negligible quantity of NH ₃ because plasma [NH ₃ ] concentration is exceedingly low. Instead, urinary NH ₃ derives mainly from diffusion into the lumen from the proximal-tubule cell (see Fig. 39-2 C ), with some entering the lumen directly via the apical Na-H exchanger NHE3. In the case of the proximal tubule, the conversion of glutamine to α-ketoglutarate (α-KG) generates two ions, which form two NH ₃ and two H ⁺ ions. In addition, the metabolism of α-KG generates two OH ⁻ ions, which CA converts to ions. This new then enters the blood. N39-3

N39-3

Ammonium Secretion by the Medullary Collecting Duct

Ammonium secretion by the medullary collecting duct is critical for renal excretion. As described in Figure 39-5 C , the TAL of juxtamedullary nephrons reabsorbs some and deposits this in the medullary interstitium, where it is partitioned between ammonium and ammonia according to the equilibrium ⇌ NH ₃ + H ⁺ . As pointed out in Figure 39-5 D , this interstitial (and NH ₃ ) can have three fates: (1) some recycles back to the late proximal tubule and descending thin limb of Henle, (2) some bypasses the cortex by being secreted into the medullary collecting duct, and (3) some is washed out by the blood for export to the liver.

The mechanism of pathway (2) is depicted in Figure 39-5 E . NH ₃ diffuses from the medullary interstitium, through the tubule cell and into the lumen. The NH ₃ moves via members of the Rh family at both the basolateral and apical membranes. The parallel extrusion of H ⁺ across the apical membrane of the collecting-duct cell provides the luminal H ⁺ that then titrates the luminal NH ₃ to , which is excreted. This luminal H pumping also generates OH ⁻ inside the cells. Although not shown in Figure 39-5 E , intracellular CA converts this newly created OH ⁻ (along with H ₂ O) to , and basolateral Cl-HCO ₃ exchangers then export this newly created to the interstitium. The , of course, ultimately is washed out by the blood. Thus, for each formed in the lumen of the collecting duct by this route, the tubule cell transfers one “new” to the blood.

Figure 39-5 E also shows that the Na-K pump can also transport directly into the collecting-duct cell. This intracellular can then dissociate into NH ₃ (which can diffuse into the lumen) and H ⁺ (which moves into the lumen via the apical H pump), with the ultimate formation of in the lumen. The that enters the collecting-duct lumen by this route does not generate a new ion.

eFigure 39-1 shows the most recent model for how the TAL handles NH ₃ and CO ₂ .

References

• Geyer RR, Musa-Aziz R, Qin X, Boron WF: Relative CO ₂ /NH ₃ selectivities of mammalian aquaporins 0-9. Am J Physiol Cell Physiol 2013; 304: pp. C985-C994.
• Weiner ID, Verlander JW: Ammonia transport in the kidney by Rhesus glycoproteins. Am J Physiol Renal Physiol 2014; 306: pp. F1107-F1120.

In summary, when renal-tubule cells secrete H ⁺ into the lumen, this H ⁺ simultaneously titrates three kinds of buffers: (1) , (2) and other buffers that become the “titratable acid,” and (3) NH ₃ . Each of these three buffers competes with the other two for available H ⁺ . In our example, the kidneys secrete 4390 mmol/day of H ⁺ into the tubule lumen. The kidneys use most of this secreted acid—4320 mmol/day or ~98% of the total—to reclaim filtered . The balance of the total secreted H ⁺ , 70 mmol/day, the kidneys use to generate new .

The nephron reclaims virtually all the filtered in the proximal tubule (~80%), thick ascending limb (~10%), and distal nephron (~10%)

The kidney reabsorbs the largest fraction of filtered (~80%) along the proximal tubule ( Fig. 39-3 A ). By the end of the proximal tubule, luminal pH falls to ~6.8, which represents only a modest transepithelial H ⁺ gradient compared with the plasma pH of 7.4. Thus, the proximal tubule is a high-capacity, low-gradient system for H ⁺ secretion. The thick ascending limb of the loop of Henle (TAL) reabsorbs an additional 10% of filtered , so that by the time the tubule fluid reaches the distal convoluted tubule (DCT), the kidney has reclaimed ~90% of the filtered . The rest of the distal nephron—from the DCT to the inner medullary collecting duct (IMCD)—reabsorbs almost all the remaining ~10% of the filtered . Although the latter portion of the nephron reabsorbs only a small fraction of the filtered , it can lower luminal pH to ~4.4. Thus, the collecting tubules and ducts are a low-capacity, high-gradient system for H ⁺ transport.

The amount of lost in the urine depends on urine pH. If the [CO ₂ ] in the urine were the same as that in the blood, and if urine pH were 5.4, the [ ] in the urine would be 0.24 mM, which is 1% of the 24 mM in blood (see p. 630 ). For a urine production of 1.5 L/day, the kidneys would excrete 0.36 mmol/day of . For a filtered load of 4320 mmol/day, this loss represents a fractional excretion of ~0.01%. In other words, the kidneys reclaim ~99.99% of the filtered . Similarly, at a nearly maximally acidic urine pH of 4.4, urine [ ] would be only 0.024 mM. Therefore, the kidneys would excrete only 36 µmol/day of filtered and would reabsorb ~99.999%.

The nephron generates new , mostly in the proximal tubule

The kidney generates new in two ways (see Fig. 39-3 B ). It titrates filtered buffers such as to produce “titratable acid,” and it titrates secreted NH ₃ to . In healthy people, excretion is the more important of the two and contributes ~60% of net acid excretion or new .

H ⁺ moves across the apical membrane from tubule cell to lumen by Na-H exchange, electrogenic H pumping, and K-H pumping

Although the kidney could, in principle, acidify the tubule fluid either by secreting H ⁺ or by reabsorbing OH ⁻ or , the secretion of H ⁺ appears to be solely responsible for acidifying tubule fluid. At least three mechanisms can extrude H ⁺ across the apical membrane; not all of these are present in any one cell.

Na-H Exchanger

Of the known NHE isoforms (see p. 124 ), NHE3 is particularly relevant for the kidney because it moves more H ⁺ from tubule cell to lumen than any other transporter. N39-4 NHE3 is present not only throughout the proximal tubule ( Fig. 39-4 A, B ) but also in the TAL (see Fig. 39-4 C ) and DCT.

N39-4

Renal NHEs

As described on page 124 of the text, several related genes encode NHEs. N5-20

In the renal proximal tubule, Na-H exchange is blocked by the removal of Na ⁺ from the lumen. Although all NHEs are far less sensitive to amiloride than the ENaC epithelial Na ⁺ channels (see pp. 758–759 and Fig. 35-4 D ), the apical NHE3 isoform in the proximal tubule is even less amiloride sensitive than the ubiquitous or “housekeeping” NHE1. The NHE1 isoform is present in the basolateral membranes of several nephron segments. The role of basolateral NHEs in acid-secreting nephron segments, such as the proximal tubule, is unclear; they may help regulate pH _i independently of transepithelial H ⁺ secretion.

Given a 10 : 1 concentration gradient for Na ⁺ from the proximal tubule lumen to the cell interior, a maximal pH gradient of 1 pH unit can be achieved by this gradient. Indeed, the late proximal tubule may have a luminal pH as low as ~6.4.

The NHE2 isoform is present at the apical membrane of the DCT, where it may participate in the apical step of H ⁺ secretion.

The apical NHE3 secretes H ⁺ in exchange for luminal Na ⁺ . Because a steep lumen-to-cell Na ⁺ gradient drives this exchange process (see p. 115 ), apical H ⁺ secretion ultimately depends on the activity of the basolateral Na-K pump.

The carboxyl termini of the NHEs have phosphorylation sites for various protein kinases. For example, protein kinase A (PKA) phosphorylates apical NHE in the proximal tubule, inhibiting it. Both parathyroid hormone and dopamine inhibit NHE3 via PKA.