Regulation of Acid-Base Balance

${\text{HCO}}_3^{-}$ Buffer System

Bicarbonate ( ${\text{HCO}}_3^{-}$ ) is an important buffer of the extracellular fluid (ECF). With a normal plasma [ ${\text{HCO}}_3^{-}$ ] of 23 to 25 mEq/L and a volume of 14 L (for a person weighing 70 kg), the ECF potentially can buffer 350 mEq of H ⁺ . The ${\text{HCO}}_3^{-}$ buffer system differs from the other buffer systems of the body (e.g., phosphate) because it is regulated by both the lungs and the kidneys. This situation is best appreciated by considering the following reaction.

As indicated, the first reaction (hydration/dehydration of CO ₂ ) is the rate-limiting step. This normally slow reaction is greatly accelerated in the presence of carbonic anhydrase (CA) . ^a

a Carbonic anhydrase actually catalyzes the following reaction: H ₂ O→H ⁺ +OH ⁻ +CO ₂ →HCO ₃ ⁻ +H ⁺ →H ₂ CO ₃

The second reaction, the ionization of carbonic acid (H ₂ CO ₃ ) to H ⁺ and ${\text{HCO}}_3^{-}$ , is virtually instantaneous.

The Henderson-Hasselbalch equation is used to quantitate how changes in CO ₂ and ${\text{HCO}}_3^{-}$ affect pH:

or

In these equations the amount of CO ₂ is determined from the partial pressure of CO ₂ (P co ₂ ) and its solubility (α). For plasma at 37°C, α has a value of 0.03. Also, pKˈ is the negative logarithm of the overall dissociation constant for the reaction in Eq. 8.1 and has a value for plasma at 37°C of 6.1. Alternatively, the relationships among ${\text{HCO}}_3^{-}$ , CO ₂ , and [H ⁺ ] can be determined as follows:

Inspection of Eqs. 8.3 and 8.4 reveals that the pH and the [H ⁺ ] vary when either the [ ${\text{HCO}}_3^{-}$ ] or the P co ₂ is altered. Disturbances of acid-base balance that result from a change in the [ ${\text{HCO}}_3^{-}$ ] are termed metabolic acid-base disorders, whereas those that result from a change in the P co ₂ are termed respiratory acid-base disorders. These disorders are considered in more detail in a subsequent section. The kidneys are primarily responsible for regulating the [ ${\text{HCO}}_3^{-}$ ] of the ECF, whereas the lungs control the P co ₂ .

Overview of Acid-Base Balance

The diet of humans contains many constituents that are either acid or alkali. In addition, cellular metabolism produces acid and alkali. Finally, alkali is normally lost each day in the feces. As described later in this chapter, the net effect of these processes is the addition of acid to the body fluids. For acid-base balance to be maintained, acid must be excreted from the body at a rate equivalent to its addition. If acid addition exceeds excretion, acidosis results. Conversely, if acid excretion exceeds addition, alkalosis results.

As summarized in Fig. 8.1 , the major constituents of the diet are carbohydrates and fats. When tissue perfusion is adequate, O ₂ is available to tissues, and insulin is present at normal levels, carbohydrates and fats are metabolized to CO ₂ and H ₂ O. Daily, 15 to 20 moles of CO ₂ are generated through this process. Normally this large quantity of CO ₂ is effectively eliminated from the body by the lungs. Therefore this metabolically derived CO ₂ has no impact on acid-base balance. CO ₂ usually is termed volatile acid because it has the potential to generate H ⁺ after hydration with H ₂ O (see Eq. 8.1 ). Acid not derived directly from the hydration of CO ₂ is termed nonvolatile acid (e.g., lactic acid).

The cellular metabolism of other dietary constituents also has an impact on acid-base balance (see Fig. 8.1 ). For example, cysteine and methionine, which are sulfur-containing amino acids, yield sulfuric acid when metabolized, whereas hydrochloric acid results from the metabolism of lysine, arginine, and histidine. A portion of this nonvolatile acid load is offset by the production of ${\text{HCO}}_3^{-}$ through the metabolism of the amino acids aspartate and glutamate. On average the metabolism of dietary amino acids yields net nonvolatile acid production. The metabolism of certain organic anions (e.g., citrate) results in the production of ${\text{HCO}}_3^{-}$ , which offsets nonvolatile acid production to some degree. Overall, in persons who ingest a diet containing meat, acid production exceeds ${\text{HCO}}_3^{-}$ production. In addition to the metabolically derived acids and alkalis, the foods ingested contain acid and alkali. For example, the presence of phosphate ( ${\text{H}}_2{\text{PO}}_4^{-}$ ) in ingested food increases the dietary acid load. Finally, some ${\text{HCO}}_3^{-}$ is normally lost in the feces. This loss is equivalent to the addition of nonvolatile acid to the body. Together, dietary intake, cellular metabolism, and fecal ${\text{HCO}}_3^{-}$ loss result in the addition of approximately 1 mEq/kg body weight of nonvolatile acid to the body each day (50 to 100 mEq/day for most adults). This acid, referred to as net endogenous acid production (NEAP) , results in an equivalent loss of ${\text{HCO}}_3^{-}$ from the body that must be replaced.

Nonvolatile acids do not circulate throughout the body but are immediately neutralized by the ${\text{HCO}}_3^{-}$ in the ECF:

This neutralization process yields the Na ⁺ salts of the strong acids and removes ${\text{HCO}}_3^{-}$ from the ECF. Thus ${\text{HCO}}_3^{-}$ minimizes the effect of these strong acids on the pH of the ECF. As noted previously, the ECF contains approximately 350 mEq of ${\text{HCO}}_3^{-}$ . If this ${\text{HCO}}_3^{-}$ was not replenished, the daily production of nonvolatile acids (≈70 mEq/day) would deplete the ECF of ${\text{HCO}}_3^{-}$ within 5 days. To maintain acid-base balance the kidneys must replenish the ${\text{HCO}}_3^{-}$ lost by neutralization of the nonvolatile acids, a process termed renal net acid excretion (RNAE) .

Renal Net Acid Excretion

Under steady-state conditions, RNAE must equal NEAP to maintain acid-base balance. Although NEAP varies from individual to individual, and from day to day in any one individual, it is not regulated. Instead the kidneys regulate RNAE to match NEAP and in so doing replenish the ${\text{HCO}}_3^{-}$ (called new ${\text{HCO}}_3^{-}$ ) lost by neutralization of the nonvolatile acids. In addition, the kidneys must prevent the loss of ${\text{HCO}}_3^{-}$ in the urine. This latter task is quantitatively more important because the filtered amount of ${\text{HCO}}_3^{-}$ is approximately 4320 mEq/day (24 mEq/L × 180 L/day = 4320 mEq/day), compared with only 50 to 100 mEq/day needed to balance NEAP.

Both the reabsorption of filtered ${\text{HCO}}_3^{-}$ and the excretion of acid are accomplished primarily by H ⁺ secretion by the nephrons. ^b

b A small amount of HCO ₃ ⁻ is reabsorbed in the proximal tubule by an apical membrane Na+-HCO ₃ ⁻ symporter (NBCn2)

Thus in a single day the nephrons must secrete approximately 4390 mEq of H ⁺ into the tubular fluid. Most of the secreted H ⁺ serves to reabsorb the filtered amount of ${\text{HCO}}_3^{-}$ . Only 50 to 100 mEq of H ⁺ , an amount equivalent to NEAP, is excreted in the urine. As a result of this acid excretion, the urine is normally acidic.

The kidneys cannot excrete urine more acidic than pH 4.0 to 4.5. Even at a pH of 4.0, only 0.1 mEq/L of H ⁺ can be excreted. Therefore to excrete sufficient acid, the kidneys excrete H ⁺ with urinary buffers such as inorganic phosphate (P _i ). ^c

c The titration reaction is HPO ₄ ⁻² +H ⁺ ↔H ₂ PO ₄ ⁻ . This reaction has a pK of approximately 6.8.

Other constituents of the urine also can serve as buffers (e.g., creatinine), although their role is less important than that of P _i . Collectively the various urinary buffers are termed titratable acid (TA) . This term is derived from the method by which these buffers are quantitated in the laboratory. Typically alkali (OH ⁻ ) is added to a urine sample to titrate its pH to that of plasma (i.e., 7.4). The amount of alkali added is equal to the H ⁺ titrated by these urine buffers and thus is termed titratable acid.

The excretion of H ⁺ as a titratable acid is insufficient to balance NEAP. An additional and important mechanism by which the kidneys contribute to the maintenance of acid-base balance is through the synthesis and excretion of ammonium ( ${\text{NH}}_4^{+}$ ) . The mechanisms involved in this process are discussed in more detail later in this chapter. With regard to the renal regulation of acid-base balance, each ${\text{NH}}_4^{+}$ excreted in the urine results in the return of one ${\text{HCO}}_3^{-}$ to the systemic circulation, which replenishes the ${\text{HCO}}_3^{-}$ lost during neutralization of the nonvolatile acids. Thus the production and excretion of ${\text{NH}}_4^{+}$ , like the excretion of titratable acid, are equivalent to the excretion of acid by the kidneys.

In brief, the kidneys contribute to acid-base homeostasis by reabsorbing the filtered ${\text{HCO}}_3^{-}$ and excreting an amount of acid equivalent to NEAP. This overall process is termed RNAE, and it can be quantitated as follows:

where $\left(U_{N{H}_4^{+}}\times \dot{V}\right)$ and $\left(U_{TA}\times \dot{V}\right)$ are the rates of excretion (mEq/day) of ${\text{NH}}_4^{+}$ and titratable acid and $\left(U_{HC{O}_3^{-}}\times \dot{V}\right)$ is the amount of ${\text{HCO}}_3^{-}$ lost in the urine (equivalent to adding H ⁺ to the body). ^d

d This equation ignores the small amount of free H ⁺ excreted in the urine. As already noted, urine with pH=4.0 contains only 0.1 mEq/L of H ⁺ .

Again, maintenance of acid-base balance means that RNAE must equal NEAP. Under most conditions, very little ${\text{HCO}}_3^{-}$ is excreted in the urine. Thus RNAE essentially reflects TA and ${\text{NH}}_4^{+}$ excretion. Quantitatively TA accounts for approximately one-third and ${\text{NH}}_4^{+}$ for two- thirds of RNAE.

${\text{HCO}}_3^{-}$ Reabsorption Along the Nephron

As indicated by Eq. 8.7 , RNAE is maximized when little or no ${\text{HCO}}_3^{-}$ is excreted in the urine. Indeed, under most circumstances, very little ${\text{HCO}}_3^{-}$ appears in the urine. Because ${\text{HCO}}_3^{-}$ is freely filtered at the glomerulus, approximately 4320 mEq/day are delivered to the nephrons and are then reabsorbed. Fig. 8.2 summarizes the contribution of each nephron segment to the reabsorption of the filtered ${\text{HCO}}_3^{-}$ .

The proximal tubule reabsorbs the largest portion of the filtered ${\text{HCO}}_3^{-}$ . Fig. 8.3 summarizes the primary transport processes involved. H ⁺ secretion across the apical membrane of the cell occurs by both a Na ⁺ -H ⁺ antiporter and H ⁺ –adenosine triphosphatase (V-type H ⁺ -ATPase). The Na ⁺ -H ⁺ antiporter (NHE3) is the predominant pathway for H ⁺ secretion (accounts for approximately two- thirds of ${\text{HCO}}_3^{-}$ reabsorption) and uses the lumen-to-cell [Na ⁺ ] gradient to drive this process (i.e., secondary active secretion of H ⁺ ). Within the cell, H ⁺ and ${\text{HCO}}_3^{-}$ are produced in a reaction catalyzed by carbonic anhydrase (CA-II). The H ⁺ is secreted into the tubular fluid, whereas the ${\text{HCO}}_3^{-}$ exits the cell across the basolateral membrane and returns to the peritubular blood. ${\text{HCO}}_3^{-}$ movement out of the cell across the basolateral membrane is coupled to other ions. The majority of ${\text{HCO}}_3^{-}$ exits through a symporter that couples the efflux of Na ⁺ with 3 ${\text{HCO}}_3^{-}$ (sodium bicarbonate cotransporter, NBC1; note: the splice variant in the kidney is NBCe-1A). Some ${\text{HCO}}_3^{-}$ exits the cell by other transporters, but they are not as important as the Na ⁺ - ${\text{HCO}}_3^{-}$ symporter NBC1. As noted in Fig. 8.3 , carbonic anhydrase (CA-IV) also is present in the brush border and basolateral membrane of the cell. The brush border enzyme catalyzes the dehydration of H ₂ CO ₃ in the luminal fluid to CO ₂ and H ₂ O, whereas the enzyme localized to the basolateral membrane facilitates the exit of ${\text{HCO}}_3^{-}$ from the cell. The movement of CO ₂ (and H ₂ O) into and out of the cell occurs via aquaporin 1 (AQP1), which is present in both the luminal and basolateral membranes. The net effect of these various transporters is that ${\text{HCO}}_3^{-}$ disappears from the luminal fluid and ${\text{HCO}}_3^{-}$ is added to peritubular blood (i.e., net ${\text{HCO}}_3^{-}$ reabsorption occurs). It has been found that a small amount of ${\text{HCO}}_3^{-}$ is reabsorbed by an apical membrane Na ⁺ - ${\text{HCO}}_3^{-}$ symporter (NBCn2).

The cellular mechanism for ${\text{HCO}}_3^{-}$ reabsorption by the thick ascending limb of the loop of Henle is similar to that in the proximal tubule. H ⁺ is secreted by an Na ⁺ -H ⁺ antiporter (NHE3) and a V-type H ⁺ -ATPase. As in the proximal tubule, the Na ⁺ -H ⁺ antiporter is the predominant pathway for H ⁺ secretion. ${\text{HCO}}_3^{-}$ exit from the cell involves both an electroneutral Na ⁺ - ${\text{HCO}}_3^{-}$ symporter (NBCn1, which transports Na ⁺ and ${\text{HCO}}_3^{-}$ in a 1:1 ratio) and a Cl ⁻ - ${\text{HCO}}_3^{-}$ antiporter (anion exchanger, AE2). Some ${\text{HCO}}_3^{-}$ may also exit the cell through Cl ^– channels present in the basolateral membrane.

The distal tubule ^e

e Here and in the remainder of the chapter we focus on the function of intercalated cells. The early portion of distal tubule, which does not contain intercalated cells, also reabsorbs HCO ₃ ⁻ . The cellular mechanism appears to involve an apical membrane Na ⁺ /H ⁺ antiporter (NHE2) and a basolateral Cl ⁻ /HCO ₃ ⁻ antiporter (AE2).

and collecting duct reabsorb the small amount of ${\text{HCO}}_3^{-}$ that escapes reabsorption by the proximal tubule and loop of Henle. Fig. 8.4 shows the cellular mechanism of H ⁺ and ${\text{HCO}}_3^{-}$ transport by intercalated cells in the distal tubule and collecting duct (see also Chapter 2 ).

One type of intercalated cell secretes H ⁺ (reabsorbs ${\text{HCO}}_3^{-}$ ) and is called the A- or α-intercalated cell . Within this cell, H ⁺ and ${\text{HCO}}_3^{-}$ are produced by the hydration of CO ₂ ; this reaction is catalyzed by carbonic anhydrase (CA-II). H ⁺ is secreted into the tubular fluid by two mechanisms. The first mechanism involves an apical membrane H ⁺ -ATPase (V-type). The second mechanism couples the secretion of H ⁺ with the reabsorption of K ⁺ via an H ⁺ -K ⁺ -ATPase similar to that found in the stomach and colon. The ${\text{HCO}}_3^{-}$ exits the cell across the basolateral membrane in exchange for Cl ⁻ (via a Cl ⁻ - ${\text{HCO}}_3^{-}$ antiporter, AE1) and enters the peritubular capillary blood. Cl ^– exit from the cell across the basolateral membrane occurs via a Cl ⁻ channel and via a K ⁺ -Cl ^– symporter (KCC4).

A second population of intercalated cells secretes ${\text{HCO}}_3^{-}$ rather than H ⁺ into the tubular fluid and is called the B- or β-intercalated cell . In these intercalated cells the H ⁺ -ATPase (V-type) is located in the basolateral membrane and a Cl ⁻ - ${\text{HCO}}_3^{-}$ antiporter is located in the apical membrane (see Fig. 8.4 ). However, the apical membrane Cl ⁻ - ${\text{HCO}}_3^{-}$ antiporter is different from the one found in the basolateral membrane of the H ⁺ -secreting intercalated cell and has been identified as pendrin . The activity of the ${\text{HCO}}_3^{-}$ -secreting intercalated cell is increased during metabolic alkalosis, when the kidneys must excrete excess ${\text{HCO}}_3^{-}$ . However, under most conditions, H ⁺ secretion predominates in these segment. ^f

f Traditionally it was believed that intercalated cells were only involved in acid-base transport. However, NaCl reabsorption is also carried out by the B-type intercalated cell. NaCl reabsorption occurs by the tandem operation of the apical membrane Cl ^−/ HCO ₃ ⁻ antiporter (pendrin) and an apical membrane Na+/Cl ⁻ 2HCO ₃ ⁻ antiporter (NDCBE). This mechanism of NaCl reabsorption is inhibited by thiazide diuretics (see Chapter 10).

The apical membrane of collecting duct cells is not very permeable to H ⁺ , and thus the pH of the tubular fluid can become quite acidic. Indeed, the most acidic tubular fluid along the nephron (pH = 4.0 to 4.5) is produced there. In comparison, the permeability of the proximal tubule to H ⁺ and ${\text{HCO}}_3^{-}$ is much higher, and the tubular fluid pH falls to only 6.5 in this segment. As explained later, the ability of the collecting duct to lower the pH of the tubular fluid is critically important for the excretion of urinary titratable acids and ${\text{NH}}_4^{+}$ .