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 Hions

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 Hions. 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 HATP 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 HPO42- and H2PO4 –. Both are concentrated in the tubular fluid because of poor reabsorption. Excess H+ ions combines with HPO42- 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 (NH) 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 HCO 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 HCO 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 NHand form NH4+. For each NH4+ excreted, one HCO3 reabsorbed in to the blood.

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