Role of kidneys in acid-base balance

1. Reabsorption of filtered bicarbonate (HCO₃⁻)
   Almost all filtered HCO₃⁻ is reabsorbed, mainly in the proximal convoluted tubule. This prevents loss of the major extracellular buffer. The collecting duct provides final adjustment of bicarbonate reabsorption.
   Brenner and Rector's The Kidney, 2-Volume Set, Acid-Base Regulation.
2. Secretion of hydrogen ions (H⁺)
   Renal tubular cells secrete H⁺ into tubular fluid, chiefly through Na⁺/H⁺ exchange in the proximal tubule and H⁺-ATPase/H⁺-K⁺-ATPase pumps in the distal nephron and collecting duct. This acidifies urine, which may reach a pH of about 4.5.
3. Excretion of acid as titratable acid
   Secreted H⁺ is buffered in urine by phosphate and other urinary buffers, then excreted as titratable acid, mainly H₂PO₄⁻. This eliminates fixed acids without an extreme fall in urine pH.
4. Ammonium (NH₄⁺) excretion
   In the proximal tubule, glutamine metabolism produces NH₄⁺ and new HCO₃⁻. NH₄⁺ is excreted in urine, while the newly formed HCO₃⁻ enters blood. Ammonium excretion rises greatly during acidosis and is the main adaptive renal mechanism for removal of excess acid.
5. Generation of new bicarbonate
   For every H⁺ excreted as titratable acid or NH₄⁺, one new HCO₃⁻ is added to the blood. This replaces bicarbonate used in buffering fixed acids.
6. Response to acid-base disturbances
   • Acidosis: increased H⁺ secretion, NH₄⁺ formation/excretion, titratable acid excretion, and HCO₃⁻ reabsorption/generation.
   • Alkalosis: decreased H⁺ secretion and HCO₃⁻ reabsorption; type B intercalated cells can secrete HCO₃⁻ into urine.

Role of Kidney in Acid-Base Balance

1. Reabsorption of bicarbonate: Almost all filtered HCO₃⁻ is reabsorbed, mainly in the proximal tubule, preventing loss of alkali.
2. Hydrogen ion secretion: Tubular cells secrete H⁺ into urine, especially in the distal tubule and collecting duct.
3. Excretion as titratable acid: H⁺ combines with urinary buffers, mainly phosphate, and is excreted as H₂PO₄⁻.
4. Ammonium excretion: Renal cells form NH₄⁺ from glutamine. NH₄⁺ is excreted in urine, allowing increased acid removal, especially in acidosis.
5. Generation of new bicarbonate: For each H⁺ excreted as titratable acid or NH₄⁺, one new HCO₃⁻ is added to blood.
6. Compensation: In acidosis, kidneys increase H⁺ and NH₄⁺ excretion and conserve HCO₃⁻. In alkalosis, they excrete more HCO₃⁻.