Renal Regulation of Acid-Base Balance

Metabolic reactions in the body produce nonvolatile acids, such as sulfuric acid, which generate an acid load of approximately 1 mEq of H⁺ per kilogram of body weight daily. Excreting H+ in the urine is essential to balance this acid load.

In the kidneys, cells within the proximal convoluted tubules (PCT) and the collecting ducts secrete hydrogen ions (H⁺) into the tubular fluid. Specifically, in the PCT, Na⁺/H⁺ antiporters secrete H⁺ while reabsorbing Na⁺.

However, the intercalated cells in the collecting duct play a more prominent role in regulating the pH of body fluids. Type A intercalated cells contain proton pumps (H⁺/ATPases) in their apical membranes that secrete H⁺ into the tubular fluid.

Interestingly, another type of intercalated cell called type B intercalated cells houses proton pumps in its basolateral membrane and Cl⁻/HCO₃⁻ antiporters in its apical membrane. These cells have a dual function: secreting HCO₃⁻ and reabsorbing H⁺. Type A intercalated cells help maintain the pH of body fluids by excreting excess H⁺ when the pH is too low, while type B intercalated cells excrete excess HCO₃⁻ when the pH is too high.

Some of the secreted H⁺ is buffered in the collecting duct by mono-hydrogen phosphate ions and a small amount of ammonia. Both of these combine with the H⁺ in the tubular fluid. The resulting dihydrogen phosphate and ammonium ions cannot diffuse back into tubule cells and are excreted in the urine.

The body ensures precise pH regulation through these coordinated mechanisms in the kidneys, safeguarding critical physiological processes.