Renal Regulation of pH

• Renal regulation of body’s acid-base balance takes place through kidney.
• It is the 3rd line of defense and long term/permanent mechanism
• Operates through 4 mechanisms:-
  • Excretion of H⁺–
  • Recovery of HCO₃^–
  • Excretion of Titratable Acids like NaH₂PO₄
  • Excretion of NH₄⁺ ions

Excretion of H⁺

• Carbonic anhydrase catalyzes formation of carbonic acid in the PCT which then dissociates into bicarbonate and H⁺
• The H⁺ then is secreted into the tubular fluid  in exchange for Na+
• The bicarbonate accompanies the Na+ into the blood and increases the alkali reserve. 

Recovery of HCO₃^–

• The h+ secreted into the lumen again forms carbonic acids which further gives C02 and H2O.
• The increase in CO2 concentration causes it to diffuse into the PCT where it recombines with water to form carbonic acid, and subsequently bicarbonate.
• Kidney reabsorbs almost all bicarboante below 25mEq/L and excretes only above it.

Excretion of Titratable Acids like NaH₂PO₄

• Secreted H+ combines with the biphosphate to form dihydrogen phosphate which is excreted in urine.
• Excretion of dihydrogen phosphate increases  in cases of acidemia.
• And decreases in case of Renal Disease with decrease in GFR.

Excretion of NH₄⁺ ions

• Deamination of glutamine produces ammonia gas in the DCT/CD.
• Glutaminase catalyzes this reaction
• Ammonia gas diffuses into the lumen and buffers with H+ to form ammonium ions which is excreted with urine.
• This excretion decreases during metabolic acidosis.