life11e_ch51

The kidneys help regulate acid–base balance

Besides regulating salt and water balance and excreting nitrogenous wastes, the kidneys have another important role: they regulate the hydrogen ion concentration (the pH) of the extracellular fluids. pH is a critical variable because it influences the structure and function of proteins.

One way to minimize pH changes in a chemical solution is to add a buffer—a substance that can either absorb or release hydrogen ions (see Key Concept 2.4). The major buffer in the blood is bicarbonate ions (HCO₃^–; see Figure 48.14) that are formed from the dissociation of carbonic acid, which in turn is formed by the hydration of CO₂ according to the following equilibrium reaction:

CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃^–

From this equation, you can see that if excess hydrogen ions are added to this reaction mixture, the reaction will move to the left and absorb the excess H⁺. If hydrogen ions are removed from the reaction mixture, however, the reaction will move to the right and supply more H⁺.

The HCO₃^– buffer system is important for controlling the pH of the blood, and therefore of the interstitial fluids as well, because the reaction can be pushed to the right and pulled to the left physiologically. The lungs control the levels of CO₂ in the blood, thus altering the acid portion of the reaction. CO₂ is considered the acid portion of the reaction because if you add additional CO₂, the reaction shifts to the right, producing more H⁺ ions. The kidneys control the base portion of the reaction by removing H⁺ from the blood and returning HCO₃^– to the blood. How does this occur?

HCO₃^– is filtered in the glomerulus and is therefore present in the tubule fluid. As illustrated in Figure 51.11A, tubule cells transport H⁺ into the tubule fluid in exchange for Na⁺. In the tubule, the excreted H⁺ combines with the filtered HCO₃^– to produce H₂CO₃ that then disassociates into H₂O and CO₂. The CO₂ diffuses into the tubule cells, where in the presence of the enzyme carbonic anhydrase it produces HCO₃^– that is transported out of the basal end of the cell into the interstitial fluid and thence to the blood. Thus for each H⁺ secreted into the tubule fluid, a HCO₃^– ion is released into the blood.

Figure 51.11 The Kidney Excretes Acids and Conserves Bases (A) Bicarbonate ions are filtered out of the blood at the glomerulus, and renal tubule cells secrete hydrogen ions into the tubule fluid. In the renal tubule, the filtered bicarbonate buffers the secreted hydrogen ions and keeps the urine from becoming too acidic. The CO₂ formed by the reaction of bicarbonate and hydrogen ions is converted back to bicarbonate by the renal tubule cells and transported back into the interstitial fluid. (B) Excretion of ammonium ions (NH₄⁺) by renal tubule cells is also important for acid–base balance.

Another mechanism for H⁺ secretion and HCO₃^– reabsorption involves ammonium ions (NH₄⁺). The metabolism of glutamine in tubule cells produces NH₄⁺ and HCO₃^– (Figure 51.11B).The HCO₃^– is reabsorbed into the interstitial fluid. The NH₄⁺ is transported into the tubule fluid and combines with Cl^–, which is excreted in the urine. This process results in the addition of a new HCO₃^– ion to the blood. The NH₄⁺ is transported into the tubules by means of an NH₄⁺ transporter.