Renal control of acid base balance

Renal control of acid base balance

The kidneys regulate acid-base balance by excreting either an acidic or basic urine. Excretion of either an acidic or a basic urine removes acids or basic from the ECF. Large numbers of HCO₃^– ions are filtered in the urine and if they are excreted into the urine, it removes base from the blood. Similarly, large numbers of H⁺­ ions are secreted into the urine, and if they are excreted, it results in loss of acid from the blood. If more HCO₃^– ions are filtered than the H⁺ secreted, there will be net loss of base from ECF.

• The kidneys regulate ECF H⁺ ions through three basic mechanisms:
  1. Tubular Secretion of H⁺ ions
  2. Reabsorption of filtered bicarbonate ions
  3. Combination of excess H⁺ ions with phosphate and amino buffers

Tubular secretion of H⁺ ions

H⁺ ions are secreted in the proximal tubule, thick ascending loop of Henle and distal tubule by sodium hydrogen counter transport. This occurs by means of active transport of sodium ions into the cell and H⁺ ions from the tubular cell into the tubular lumen against the concentration gradient provided by sodium-potassium ATP pump. For each H⁺ ion secreted, one HCO₃^– ion is reabsorbed. When CO₂ diffuses into the tubular cells,  formed by metabolism, CO₂ combines with water, forms H₂CO₃ which dissociates into HCO₃^– and H⁺ ions. H⁺ ions are secreted from the cell into the lumen by sodium-hydrogen counter transport.

The sodium moves into the cell by concentration gradient established by sodium-potassium ATPase pump in the basolateral membrane. H⁺ ions are also secreted in the distal tubule and collecting ducts and transported through H⁺ pump by H⁺ ATP ase mechanism.

Reabsorption of filtered bicarbonates

The filtered bicarbonate ions are not easily reabsorbed across the tubular membrane. The bicarbonates combines with H⁺ ions to form H₂CO₃ which then dissociates to form Co₂ and H₂O. This CO₂ moves across the tubular membrane and diffuses immediately into the tubular cell.

Inside the cell, it combines with H₂O to from H₂CO₃ in the presence of carbonic anhydrase and H₂Co₃­ dissociate to HCO₃^– and H⁺ ion. Therefore for energy H⁺ ion formed in the tubular epithelial cells, a HCO₃^– ion is formed and reabsorbed into the blood.

Combination of excess H⁺ with phosphate buffer

When excess of H⁺ ions are secreted, only a small fraction of is excreted in the ionic form (H⁺) in the urine and the remaining H⁺ ions combines with buffers such as phosphate and ammonia buffer in the tubular fluid as urine can be acidified to a pH of about 4.5.

The phosphate buffer system is composed of HPO₄^2- and H₂PO₄ ^–. Both are concentrated in the tubular fluid because of poor reabsorption. Excess H⁺ ions combines with HPO₄^2- to form H₂PO₄ which in turn are excreted as sodium salt (NaH₂PO₄).

Combination of excess H⁺ with ammonia buffer system

This buffer system is composed of ammonia (NH₃ ) and ammonium ion (NH₄⁺). Ammonium ion is synthesized from glutamine which is actively transported into the tubular epithelial cells. Inside the cell, glutamine is metabolised to form NH₄⁺ and two HCO₃ ^– ions. The NH₄⁺ is secreted into the tubular lumen by countercurrent mechanism in exchange for sodium which is reabsorbed. The HCO₃^– moves across the basolateral membrane along with reabsorbed Na⁺ into the blood. Therefore for each molecule of glutamine metabolised in the proximal tubule, two NH₄⁺ ions are secreted into the urine and two HCO₃ ^– ions are reabsorbed into the blood.

In the collecting tubule, formation of NH₄⁺ ions occurs by a combination of NH₃ (ammonia) with H⁺ ions and then excreted as NH₄⁺. The collecting ducts are permeable to NH₃ and form NH₄⁺_. Hydrogen ions  react with NH₃ and form NH₄^+. For each NH₄⁺ excreted, one HCO₃^– reabsorbed in to the blood.