Renal regulation of acid base balance:

An important function of kidney is to maintain pH of the extracellular fluid. It does so by excreting acids and bases in the urine.  Excretion of acids in the urine reduces the acids of blood and excretion of bases reduces the bases of the blood.

The major renal mechanisms for regulation of pH are:

1. Excretion of hydrogen ion
2. Excretion of ammonium ion
3. Reabsorption of bicarbonate
4. Excretion of some acids.

Excretion of hydrogen ions:

This process occurs in proximal convoluted tubules. The CO₂ combines with H₂O to form H₂CO₃ (carbonic acid) in presence of carbonic anhydrase. The H₂CO₃ then ionizes into H+ and bicarbonate (HCO₃-). These hydrogen ions are secreted into tubular lumen in exchange with sodium ions. From tubular lumen the hydrogen ions are then excreted. This mechanism helps to increase alkali (bicarbonate ions) and urine becomes acidic.

Excretion of ammonium ion:

This occurs in DCT. the glutamine from tubular lumen is converted into ammonia and glutamic acid. The ammonia thus formed can bind with the hydrogen ion to form ammonium ion. Now this ammonium ion is excreted.

Reabsorption of bicarbonate:

This occurs in PCT and this process helps in conserving the base. The bicarbonate ion in the tubular lumen combines with the hydrogen ion to form carbonic acid. It dissociates into carbon dioxide and again diffuses into the cell and again combines with water to form carbonic acid.

Excretion of some acids:

It occurs in DCT. Sometimes the hydrogen ion combines with the basic phosphates to form acid phosphates.  The acid and base phosphate thus formed can act as buffer in the renal tubules and considered as urinary buffers.