When fewer hydrogen ions are secreted into the filtrate, how is blood ph affected?

A simplified overview of the renal processing of acids and bases is as follows: The acid-base component of highest concentration in the blood is bicarbonate (24 – 28 mEq/L), and even under unusual circumstances, the kidneys have to reabsorb most of the filtered load. This is accomplished primarily in the proximal tubule, thus conserving plasma bicarbonate. The proximal tubule also secretes limited amounts of organic bases or weak organic acids and acid equivalents as previously described in Chapter 5. Then, in the distal nephron (mostly the collecting tubules), the kidneys secrete either protons or bicarbonate to balance the net input into the body (summarized in Table 9–1).

Table 9–1.Normal contributions of tubular segments to renal hydrogen ion balance

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Table 9–1. Normal contributions of tubular segments to renal hydrogen ion balance

Proximal tubule Reabsorbs majority of filtered bicarbonate (normally about 80%) Produces and secretes ammonium

Thick ascending limb of Henle's loop Reabsorbs most of remaining filtered bicarbonate (normally about 10%–15%)

Distal nephron Reabsorbs virtually all remaining filtered bicarbonate as well as any secreted bicarbonate (Type A intercalated cells) Acidifies tubular fluid (Type A intercalated cells) Secretes bicarbonate (Type B intercalated cells) Secretes ammonia and ammonium (type A and non-A, non-B intercalated cells)

Reabsorption of Bicarbonate

The first task is always to reabsorb most of the filtered bicarbonate. Bicarbonate is freely filtered at the renal corpuscles. Reabsorption is an active process, but it is not accomplished in the conventional manner of importing bicarbonate across the apical membrane and exporting it across the basolateral membrane. Rather, the mechanism is an acid-base process involving the secretion of hydrogen ions. Since each of the many liters of filtrate contains bicarbonate in the range of 24 to 28 mEq/L, an enormous amount of hydrogen ion secretion occurs in the proximal tubule. The basic principle is the same as secretion in the GI tract illustrated in the upper part of Figure 9–1. Within the tubular cells, hydrogen ions and bicarbonate are generated from CO2 and water, catalyzed by carbonic anhydrase. The hydrogen ions are actively secreted into the tubular lumen in exchange for sodium via an antiporter. Those hydrogen ions combine with filtered bicarbonate to form water and carbon dioxide; thus, the filtered bicarbonate “disappears.” At the same time, the cellular bicarbonate is transported across the basolateral membrane into the interstitial fluid and then into the peritubular capillary blood. The overall result is that the bicarbonate filtered from the blood at the renal corpuscle is converted to CO2 and water, and is replaced by bicarbonate generated inside the cell. Thus, no net change in plasma bicarbonate concentration occurs. It is also important to note that the hydrogen ion that was secreted into the lumen is not excreted in the urine. It has been incorporated into water. Any secreted hydrogen ion that combines with bicarbonate in the lumen does not contribute to the urinary excretion of hydrogen ions, but only to the conservation of bicarbonate.

Bicarbonate is reabsorbed by combining it with secreted hydrogen ions, turning it into CO2 and water, while simultaneously generating intracellular bicarbonate and transporting it to the interstitium.

Specific transporters are required for these transmembrane movements of hydrogen ions and bicarbonate. Particularly prominent in the apical membrane of the proximal tubule is the Na-H antiporter (NHE3) as described in Chapter 4 and shown in Figure 9–2. This transporter is the major means not only of hydrogen ion secretion but also of sodium uptake from the proximal tubule lumen. The same NHE3 antiporter also mediates hydrogen ion secretion in the thick ascending limb. In the distal nephron segments that secrete hydrogen ions there are primary active H-ATPases. The Type A intercalated cells of the collecting-duct system possess this primary active H-ATPase as well as a primary active H-K-ATPase, which simultaneously moves hydrogen ions into the lumen and potassium into the cell, both actively (Figure 9–3A).

Figure 9–2.

Predominant proximal tubule mechanism for reabsorption of bicarbonate. Hydrogen ions and bicarbonate are produced intracellularly. The hydrogen ions are secreted via a Na-H antiporter (member of the NHE family), whereas the bicarbonate is transported into the interstitium via an Na-3HCO3 symporter (member of the NBC family). Because more sodium enters via the Na-H antiporter than leaves via the Na-3HCO3 symporter, additional sodium is removed via the Na-K-ATPase.

Figure 9–3.

Type A and type B intercalated cells. A, Predominant mechanisms in Type A intercalated cells for the secretion of hydrogen ions that result in formation of titratable acidity. The apical membrane contains H-ATPases and H-K-ATPases, which transport hydrogen ions alone or in exchange for potassium. Bicarbonate moves across the basolateral membrane predominantly via the AE1 antiporter. B, The type B intercalated cell secretes bicarbonate via the pendrin antiporter and simultaneously transports hydrogen ions into the interstitium.

The basolateral membrane exit step for bicarbonate generated when hydrogen ions are secreted is via Cl–HCO3 antiporters or Na-HCO3 symporters (Figures 9–2 and 9–3A), depending on the tubular segment. In both cases, the movement of bicarbonate is down its electrochemical gradient (ie, the exit step is passive). Symport with sodium is the dominant means of extruding bicarbonate in the proximal tubule. This process is particularly interesting because the efflux of sodium is up its electrochemical gradient. This is a rare case of sodium active transport that does not use ATP as the energy source, but uses the gradient of another ion. (However, this process can only occur if the Na-K-ATPase sets up the sodium gradient that powers the removal of hydrogen ions via Na-H exchange in the apical membrane.)

Through its secretion of hydrogen ions, the proximal tubule reabsorbs 80% to 90% of the filtered bicarbonate. The thick ascending limb of Henle's loop reabsorbs another 10%, and almost all the remaining bicarbonate is reabsorbed in the distal nephron (although this depends on diet and other conditions; see later discussion).

Throughout the tubule, intracellular carbonic anhydrase is involved in the reactions generating hydrogen ion and bicarbonate. In the proximal tubule, carbonic anhydrase is also located in the lumen-facing surface of apical cell membranes, and this carbonic anhydrase catalyzes the intraluminal generation of CO2 and water from the large quantities of secreted hydrogen ions combining with filtered bicarbonate.

Acid or base loads, regardless of original source, are turned into an excess or deficit of bicarbonate.

Acid and Base Excretion

Acid or base loads generated from the processes described earlier result in changes in plasma bicarbonate. In essence, an acid or base load, regardless of original source, is turned into an excess or deficit of bicarbonate. The task of the kidneys is to excrete the excess or replace the deficit.3 In response to base loads the process is relatively straightforward: The kidneys reabsorb most of the filtered bicarbonate, but excrete just enough bicarbonate in the urine to match the input. The kidneys do this in 2 ways: (1) allow some filtered bicarbonate to pass through to the urine and (2) secrete bicarbonate via Type B intercalated cells in the distal nephron. The Type B intercalated cell reverses the location of the relevant transporters found in the type A intercalated cell (Figure 9–3B). Within the cytosol, hydrogen ions and bicarbonate are generated via carbonic anhydrase. However, the H-ATPase transporter is located in the basolateral membrane, and the Cl-HCO3 antiporter, called pendrin, is in the apical membrane. Accordingly, bicarbonate moves into the tubular lumen via pendrin, and hydrogen ions are actively transported out of the cell across the basolateral membrane and enter the blood, where they combine with bicarbonate ions and reduce plasma bicarbonate. Thus, the overall process achieves the disappearance of excess plasma bicarbonate and the excretion of bicarbonate in the urine.

How do the kidneys excrete an acid load, that is, replace a bicarbonate deficit? First, be aware that generating bicarbonate from CO2 and water simultaneously generates hydrogen ions. The hydrogen ions must be separated from the bicarbonate and excreted, otherwise these components just recombine and accomplish nothing. The process begins by reabsorbing all the filtered bicarbonate. Then the kidneys secrete additional hydrogen ions (via H-ATPases in type A intercalated cells) that attach to bases in the tubular fluid other than bicarbonate. The now protonated base is excreted. Simultaneously, the bicarbonate generated in the intercalated cell is transported across the basolateral membrane into the blood via Cl-HCO3 antiporters called AE1, replacing the bicarbonate lost when the acid load entered the body. We emphasize again that both parts of this process must occur, that is, generation of new bicarbonate and excretion of hydrogen ions on nonbicarbonate bases. If there were no new bicarbonate, plasma levels would not be restored, and if hydrogen ions were not excreted, they would recombine with the bicarbonate just generated.

3Acid-base loads are partly buffered by nonbicarbonate blood buffers and intracellular buffers. Eventually, however, those buffers must release the loads they have taken up and this again turns into changes in plasma bicarbonate.

Hydrogen Ion Excretion on Urinary Bases

We see that the identical transport process of hydrogen ion secretion achieves both reabsorption of bicarbonate (without new bicarbonate), and acid excretion, with addition of new bicarbonate to the blood. At first glance, this seems like a contradiction: How can the same process produce 2 different results? The answer lies in the fate of the hydrogen ion once it is in the lumen. For secreted hydrogen ions that combine with bicarbonate (a process that always occurs in the proximal tubule), we are simply replacing filtered bicarbonate that would have left the body. The hydrogen ion is incorporated into water. In contrast, when secreted hydrogen ions combine with a nonbicarbonate base in the lumen of the distal nephron, where little if any bicarbonate remains, the hydrogen ion is excreted and the bicarbonate produced in the cell and transported across the basolateral membrane is new bicarbonate, not a replacement for filtered bicarbonate.

Phosphate and Organic Anions

There are 2 sources of urinary nonbicarbonate bases: filtration and synthesis. Normally, the most important filtered base is phosphate, whereas ammonia is the most important synthesized base. Free plasma phosphate exists as a mixture of monovalent (acid) and divalent forms (base). As shown in Equation 9-5, monovalent dihydrogen phosphate (on the left) is a weak acid, and divalent monohydrogen phosphate (on the right) is its conjugate base.

Equation 9-5H2PO4−↔HPO42−+ H+

We can write this in the form of the Henderson-Hasselbalch equation:

Equation 9-6pH=6.8+log[HPO42− ]/[H2PO4−]

At the normal plasma pH of (7.4), we find that about 80% of the plasma (and filtered) phosphate is in the base (divalent) form and 20% is in the acid (monovalent) form. Much of the filtered phosphate is reabsorbed in the proximal tubule, but the rest flows on to the distal nephron. As hydrogen ions are secreted in the collecting ducts and tubular pH falls, the remaining base form takes up secreted hydrogen ions. Depending on final urine pH, the majority of the base (HPO42−) is protonated to acid (H2PO4−). The secreted hydrogen ions that combined with the base form are excreted, and the bicarbonate that was generated intracellularly enters the blood. How much phosphate is available for this process? The amount is somewhat variable, depending on a number of factors, but a typical plasma concentration is about 1 mmol/L, of which about 90% is free (the rest being loosely bound to plasma proteins). At a GFR of 180 L/day, the total filtered load of phosphate is about 160 mmol/day. The fraction reabsorbed is also variable: from 75% to 90%. Thus, unreabsorbed divalent phosphate available to take up secreted hydrogen ions amounts to roughly 40 mmol/day. In other words, the kidneys can excrete acid loads using the filtered phosphate at a rate of about 40 mmol/day. Figure 9–4 illustrates the sequence of events that achieves hydrogen ion excretion on filtered phosphate and the addition of new bicarbonate to the blood.

Figure 9–4.

Excretion of hydrogen ions on filtered phosphate. Divalent phosphate (base form) that has been filtered and not reabsorbed reaches the collecting tubule, where it combines with secreted hydrogen ions to form monovalent phosphate (acid form), which is then excreted in the urine. The bicarbonate entering the blood is new bicarbonate, not merely a replacement for filtered bicarbonate.

Acid excretion simultaneously regenerates bicarbonate.

It must be emphasized also that neither filtration nor excretion of free hydrogen ions makes a significant contribution to hydrogen ion excretion. First, the filtered load of free hydrogen ions, when the plasma pH is 7.4 (40 nmolar/H+), is less than 0.1 mmol/day. Second, there is a minimum urinary pH—approximately 4.4—that can be achieved. This corresponds to a free hydrogen ion concentration of 0.04 mmol/L. With a typical daily urine output of 1.5 L, the excretion of free hydrogen ions, even at the most acidic pH, could only be 0.06 mmol/day, a tiny fraction of the normal 50 to 100 mmol of hydrogen ion ingested or produced every day. To excrete these additional amounts of protons, they must associate with tubular bases.

Hydrogen Ion Excretion as Ammonium

Ordinarily, hydrogen ion excretion associated with phosphate and other filtered bases is not sufficient to balance the normal hydrogen ion production of 50 to 100 mmol/day, nor can it take care of any unusually high production of acid loads. To excrete the rest of the hydrogen ions and achieve balance, there is a second means of excreting hydrogen ions that involves ammoniagenesis and excretion of hydrogen ions as ammonium. Quantitatively, far more hydrogen ions can be excreted by means of ammonium than via filtered bases. Furthermore, although the amount of filtered base cannot be changed to serve the needs of acid-base balance, ammoniagenesis can be greatly increased in response to high acid loads. There are many nuances to hydrogen ion excretion via ammonium, but the basic concepts are straightforward.

As described in Chapter 5, the catabolism of protein and oxidation of the constituent amino acids by the liver generates CO2, water, urea, and some glutamine. Although the metabolism of the side chains of amino acids can lead to addition of acid or base, the processing of the core of an amino acid—the carboxyl group and amino group—is acid-base neutral. After many intermediate steps, processing of the carboxyl group of the amino acid produces bicarbonate, and processing of the amino group produces ammonium (NH4+), which is the protonated form of ammonia (NH3). Processing does not stop there, however, because ammonium in more than miniscule levels is quite toxic. Ammonium is further processed by the liver to either urea or glutamine. In both cases, each ammonium consumed also consumes a bicarbonate. Thus, the bicarbonate produced from the carboxyl group is just an intermediate, consumed as fast as it is made, and the process as a whole is acid-base neutral. We can write this process schematically as follows:

Equation 9-72 amino acids→2NH4++2HCO3−→urea or glutamine

When either urea or glutamine is excreted, the body has completed the catabolism of protein in a manner that promotes total body nitrogen balance, and is acid-base neutral.

The renal handling of urea is somewhat complicated from the osmotic point of view, as described in earlier chapters, but is acid-base neutral. The renal handling of glutamine, however, is different. Although the production of glutamine by the liver is acid-base neutral, it is helpful to recognize that glutamine can be thought of as containing the 2 components from which it was synthesized: a base component (bicarbonate) and an acid component (ammonium). Ammonium is an acid because it contains a dissociable proton as shown in Equation 9-8. The pK of ammonium is near 9.2, making it an extremely weak acid (ie, only at high pH will it release its proton), but it is an acid nevertheless. At physiological pH, over 98% of the total exists as ammonium, and less than 2% exists as ammonia. For renal acid-base purposes, this is a good thing because virtually all excreted ammonia is in the protonated form and takes a hydrogen ion with it.

Glutamine released from the liver is taken up by proximal tubule cells, both from the lumen (filtered glutamine) and from the renal interstitium via Na-glutamine symporters. The cells of the proximal tubule then convert the glutamine back to bicarbonate and NH4+, in essence reversing what the liver has done. The NH4+ is secreted into the lumen of the proximal tubule, and the bicarbonate exits into the interstitium and then into the blood (Figure 9–5). This is new bicarbonate, just like the new bicarbonate generated by titrating nonbicarbonate bases. Further processing of the NH4+ is complicated, but eventually the ammonium is excreted (Figure 9–6).

Figure 9–5.

Ammoniagenesis and excretion. Ammonium production from glutamine. Glutamine is originally synthesized in the liver from NH4+ and bicarbonate. When glutamine reaches the proximal tubule cells, it is converted via several intermediate steps (not shown) back to NH4+ and bicarbonate. The bicarbonate is transported into the blood and the ammonium is secreted.

Figure 9–6.

Ammonium secretion in the inner medulla. Several mechanisms are involved. A prominent one involves uptake and secretion of neutral ammonia via specific transporters in parallel with hydrogen ion secretion, resulting in reformation of ammonium in the lumen. In the innermost medulla, the high interstitial ammonium concentration allows ammonium to substitute for potassium on the Na-K-ATPase.

The ammonium ion has interesting chemical properties in that it can “masquerade” as other ions, in some cases as a hydrogen ion and in other cases as a potassium ion. This is because some transporters and some channels are not completely selective for the species they usually move compared to ammonium. As the concentration of ammonium rises, there is an increasing tendency for ammonium to substitute for these other ions and “sneak” its way across membranes.

Large acid loads are excreted mainly in the form of ammonium.

Also, whenever ammonium is present in body fluids, a small fraction (2% at physiological pH) always exists as ammonia because the dissociation, although limited in extent, is nearly instantaneous. Ammonium, being a small hydrated ion, is essentially impermeant in lipid bilayers and must be handled by channels or transporters if it is to move across membranes. The neutral ammonia has low but finite lipid bilayer permeability. More importantly, there are uniporters for ammonia, members of the Rh glycoprotein family, which transport ammonia in some regions of the nephron. In terms of cellular handling, cells sometimes transport ammonium as such and at other times transport ammonia and a proton in parallel, the end result being the same in both cases.

It would “make sense” if the ammonium secreted into the proximal tubule simply stayed in the lumen and was excreted, but the kidneys have evolved a more complicated way of doing things. An array of transporters participate in moving ammonium or ammonia into or out of the tubule in various segments. So long as all the ammonium produced from glutamine and secreted in the proximal tubule ends up being excreted, the process accomplishes the goal of excreting acid, even if ammonium is transported as such in some places and moved as H+ and NH3 separately in other places. But if ammonium is returned to the circulation, it is metabolized by the liver back to urea, consuming bicarbonate in the process, thereby nullifying the renal generation of bicarbonate.

Most of the ammonium synthesized from glutamine in the proximal tubule is secreted via the NHE3 antiporter in exchange for sodium, with ammonium substituting for a hydrogen ion (Figure 9–5). The next major transport event occurs in the thick ascending limb. In this segment approximately 80% of the tubular ammonium is reabsorbed, mostly by the Na-K-2Cl multiporter, with ammonium now substituting for potassium, and exits via an antiporter in exchange for sodium (Figure 9–7). In the medullary portions of the thick ascending limb, this reabsorption results in accumulation of ammonium (and therefore some ammonia) in the interstitium, with the concentration progressively increasing towards the papilla, analogous to the osmotic gradient. Finally, in the medullary collecting ducts, there is secretion once again. Ammonia is taken up from the interstitium via Rh glycoprotein uniporters, and ammonium in taken up via the Na-K-ATPase, with ammonium substituting for potassium. Ammonia exits into the lumen via an apical Rh glycoprotein and combines with a hydrogen ion secreted via an H-ATPase (Figure 9–6). Thus the ammonium that was reabsorbed in the thick ascending limb and accumulated in the medullary interstitium is now put back into the tubule and excreted. The processes of excreting acid and base are summarized in Figure 9–8.

Figure 9–7.

Ammonium reabsorption in the thick ascending limb. Ammonium reaches the thick ascending limb from 2 sources. Most comes as a result of secretion in the proximal tubule. Some also enters the thin limbs from the medullary interstitium in the form of neutral ammonia and is subsequently reprotonated in the lumen (ammonium recycling). Ammonium is reabsorbed in the thick ascending limb by several mechanisms, the predominant one being entrance via the NKCC multiporter (where ammonium substitutes for potassium) and exit via a sodium–ammonium antiporter (NHE-4).

Figure 9–8.

Overall scheme for excretion of acid and base. In all cases, the majority of filtered bicarbonate is reabsorbed. For base excretion, some filtered bicarbonate moves through to be excreted along with some additional bicarbonate secreted by type B intercalated cells in the distal nephron. For acid excretion all the filtered bicarbonate is reabsorbed. Then hydrogen ions are secreted in the distal nephron, contributing to titratable acidity. Ammonium excretion accounts for the bulk of acid excretion.

Quantification of Renal Acid-base Excretion

We can quantify the excretion of acid/base equivalents by looking at 3 quantities in the urine: (1) the amount of titratable acidity, (2) the amount of ammonium, and (3) the amount of bicarbonate, if any. Titratable acidity represents the amount of acid that was taken up by urinary bases other than ammonia. It can be measured by titrating the urine with strong base (NaOH) to a pH of 7.4. (The amount of NaOH required to increase the pH back to 7.4 thus equals the amount of hydrogen ion that was secreted and combined with phosphate and organic bases). Urinary ammonium equals the urinary volume times the urinary ammonium concentration. (Ammonium does not contribute to titratable acidity because with a pK of 9.2, few hydrogen ions are removed by titration to pH 7.4). Similarly, urinary bicarbonate equals the urinary volume times the urinary bicarbonate concentration.

Thus, we can write the net acid excretion as:

Equation 9-9Netacidexcretion =titratableacidexcreted+NH4+excreted−HCO3−excreted

Note that there is no term for free hydrogen ion in the urine because, even at a minimum urine pH of 4.4, the number of free hydrogen ions is trivial.

Typical urine data for the amounts of bicarbonate contributed to the blood by the kidneys in 3 potential acid-base states are given in Table 9–2. Note that in response to acidosis, as emphasized previously, increased production and excretion of NH4+ is quantitatively much more important than increased formation of titratable acid.

Table 9–2.Renal contribution of new bicarbonate to the blood in different states

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Table 9–2. Renal contribution of new bicarbonate to the blood in different states

AlkalosisNormal stateAcidosis

Titratable acid (mmol/day)	0	20	40
plus NH4+ excreted (mmol/day)	0	40	160
minus HCO3− excreted (mmol/day)	80	1	0
Total added to body (mmol/day)	−80	59	200
Urine pH	8.0	6.0	4.6

When fewer hydrogen ions are secreted into the filtrate How is blood pH affect?

What happens to serum pH when excess hydrogen ions accumulate in the blood?

What happens when extra hydrogen ions are added to the blood?

What is the pH range resulting from a normal hydrogen ion concentration of body fluids?