Question

what two processes might be occurring in specific sections of the renal tubules in order to correct for pH imbalance.?

Answer 1

Renal compensation of pH imbalances involve two processes linked to excretion or reabsorption of bicarbonate and H+ ions.

Excretion of H+ ions: Hydrogen ions are excreted in form of dihydrogen phosphate. The hydrogen ATPase pump present on intercalated cells in distal convoluted tubule and collecting ducts. This pump transports hydrogen ions, while ATPase provides the energy for transport. H+ ions bind to phosphate ions to form dihydrogen phosphate for excretion. Hydrogen ions are also excreted in from of ammonia in proximal convoluted tubules. The amino acid glutamine is converted to glutamate and hydrogen ions. Ammonia move via the membrane into lumen where it binds to a luminal H+ ions. Ammonium formed will be excreted in urine, thereby causing H+ ions secretion. Ammonia production also helps to buffer the urine pH. Ammonia can pass into the collecting duct from the proximal tubule, where it can bind to luminal H+ ions to form ammonium. Thus, it helps in excretion of H+ ion by collecting duct intercalated cells

This excretion of H+ ions occurs when the pH of blood decreases. The loss of H+ ions via urine will cause the pH of blood to increase. Conversely, if pH of blood increases, the secretion of H+ ions will decrease, conserving H+ ions and increasing the pH of blood.

Reabsorption of Bicarbonate: The kidneys maintain an acid-base balance by controlling bicarbonate reabsorption. It also replaces the bicarbonate lost by generating new bicarbonate ions. Most of the filtered bicarbonate (80%) is reabsorbed in the proximal tubule of the Kidney. 15% of the remaining bicarbonate is reabsorbed in the thick ascending loop while rest 5% is reabsorbed in distal convoluted tubule.

In the proximal convoluted tubule of kidneys, the sodium hydrogen exchanger will cause H+ ions to be secreted in lumen of the tubule. There is transport of H+ ions cell by this pump in exchange for sodium ions. The H+ ions react with bicarbonate ions in lumen to form carbonic acid (H2CO3). This reaction is catalyzed by carbonic anhydrase enzyme in the lumen. Carbonic acid then dissociates to CO2 and water. Both CO2 and water can easily pass through the cell membrane into PCT cells, where they recombine to form carbonic acid. Carbonic acid dissociates to form H+ and HCO3-. Carbonic anhydrase present in the cell carries out these reactions. Bicarbonate ions are then reabsorbed into blood. Sodium ions will also be exchanged for H+ ions from blood, which are the transported to the lumen, causing repetition of the cycle. There can also be production of CO2 by the kidney cells. These along with water form bicarbonate and H+ ions. Bicarbonate can then be reabsorbed into the blood along with chloride ions, while sodium is exchanged with H+ ions. Thus, bicarbonate along with sodium enters the blood via the sodium-potassium co-exchanger. Kidneys correct increase in blood pH by decreasing bicarbonate reabsorption and increasing bicarbonate secretion. They decrease blood pH by decreasing excretion of bicarbonate and increasing bicarbonate reabsorption.

Kidneys also produce bicarbonate ions via excretion of acid into urine. This net acid excretion is carried out by excretion of titratable acid (two-thirds) and ammonium (NH4+) (one third). When the pH of blood decreases (higher H+), there is loss of bicarbonate in urine. This loss of bicarbonate causes generation of HCO3- and net excretion of acid to decrease in the kidneys.