H 2 O to form H 2 CO 3 that dissociatesto form H + and HCO 3– .

membrane

Tubule cellcytoplasm 3 An antiport mechanism secretes H + intothe filtrate in exchange for Na + from the Apicalfiltrate. As a result, filtrate pH decreases. membrane

4 Bicarbonate ions are symported withNa + into the interstitial fluid. The HCO 3– then diffuses into capillaries.

5 In capillaries, HCO 3– combines with H + .This decreases the H + concentrationand increases blood pH.

Lumen

CO 2 + H 2 O

2

H 2 CO 3

H + + HCO 3

–

Na +

4

PROCESS FIGURE 27.14 Kidney Regulation of Body Fluid Acid-Base Balance

As the extracellular pH decreases, the rate of H + secretion by the renal tubule cells and HCO 3 − reabsorption increase.

increases also. As H 2 CO 3 increases, many H 2 CO 3 molecules dis-sociate to form H + and HCO 3− . This results in an elevated H + concentration, and the pH decreases toward its normal range.

Predict 3

Under stressful conditions, some people hyperventilate. What effect doesthe rapid rate of ventilation have on blood pH? Explain why breathing intoa paper bag helps a hyperventilating person.

Renal Regulation of Acid-Base Balance

Cells of the kidney tubules can directly regulate acid-base balanceby changing both the rate of H + secretion into the filtrate and therate of HCO 3− reabsorption (figure 27.14). Carbonic anhydrasewithin tubule cells catalyzes the formation of H 2 CO 3 from CO 2 and H 2 O. The carbonic acid molecules dissociate to form H + andHCO 3− . An antiport system then exchanges H + for Na + across theapical membrane of the cells. Thus, tubule cells secrete H + into thefiltrate and reabsorb Na + . The Na + and HCO 3− are symportedacross the basal membrane. After the Na + and HCO 3− are sym-ported from the tubule cells, they diffuse into the peritubularcapillaries. As a result, H + is secreted into the  lumens of thetubules, and HCO 3− passes into the extracellular fluid.The reabsorbed HCO 3− combines with excess H + in the extra-cellular fluid to form H 2 CO 3 . This combination removes H + fromthe extracellular fluid and increases extracellular pH. The rate ofH + secretion and HCO 3− reabsorption increases when the pH of thebody fluids decreases, and this process slows when the pH of thebody fluids increases (figure 27.14).Some of the H + secreted by cells of the tubules into the filtratecombines with HCO 3− , which enters the filtrate through the filtra-tion membrane in the form of sodium bicarbonate (NaHCO 3 ).

1022

Within the tubules, H + combines with HCO 3− to form H 2 CO 3 ,which then dissociates to form CO 2 and H 2 O. The CO 2 diffusesfrom the filtrate into the tubule cells, where it can react with H 2 Oto form H 2 CO 3 , which subsequently dissociates to form H + andHCO 3− (figure 27.15). Once again, H + is antiported into thelumens of the kidney tubules in exchange for Na + , whereas HCO 3− enters the extracellular fluid. As a result, much of the HCO 3− entering the filtrate reenters the extracellular fluid.Hydrogen ions secreted into the tubule normally exceed theamount of HCO 3− that enters the tubules through the filtrationmembrane. Because the H + combines with HCO 3− , almost all theHCO 3− is reabsorbed from the tubules (figure 27.15). LittleHCO 3− is excreted in the urine unless the pH of the body fluidsbecomes elevated.The rate of H + secretion into the filtrate and the rate of HCO 3− reabsorption into the extracellular fluid decrease if the pH of thebody fluids increases. As a result, the amount of bicarbonate fil-tered into the tubules exceeds the amount of secreted H + , and theexcess HCO 3− passes into the urine. The excretion of excessHCO 3− in the urine diminishes the amount of HCO 3− in the extra-cellular fluid. This allows the amount of extracellular H + toincrease; as a consequence, the pH of the body fluids decreasestoward its normal range.Although the tubule cells respond directly to H + , the hormonealdosterone can also alter the H + permeability of the tubules.Aldosterone increases the rate of Na + reabsorption and K + secre-tion by the kidneys, but in high concentrations aldosterone alsostimulates H + secretion. Elevated aldosterone levels, such as thoseoccurring in patients with Cushing syndrome, can therefore ele-vate body fluid pH above normal (alkalosis). However, the majorinfluence on the rate of H + secretion is the pH of the bodyfluids.