Ammonia Mechanism in Applied Physiology - Acid-Base Balance

AMMONIA MECHANISM IN APPLIED PHYSIOLOGY

ACID-BASE BALANCE

• Role of the Kidney:

  • The kidneys play a crucial role in the maintenance of acid-base balance by:
  • Excreting hydrogen ions (H+).
  • Retaining bicarbonate ions (HCO₃⁻).

• pH of Urine:

  • Urine typically exhibits acidic nature with a pH range of 4.5 to 6.
  • It contains a large quantity of acids, notably hydrogen ions.

• Metabolic Activities:

  • Various metabolic activities in the body can threaten to induce acidosis.

Mechanisms to Prevent Acidosis

1. Reabsorption of Bicarbonate Ions (HCO₃⁻):

   • Approximately 4,320 mEq of HCO₃⁻ is filtered by the glomeruli daily, referred to as the filtered load of HCO₃⁻.
   • Excretion of this amount of HCO₃⁻ in urine would significantly impact acid-base balance; hence, reabsorption is vital.

2. Secretion of Hydrogen Ions (H+):

   • 4,380 mEq of H+ appear daily in the renal tubule through filtration and secretion.
   • Not all of this H+ is excreted; approximately 4,280 to 4,330 mEq of H+ is used for the reabsorption of filtered HCO₃⁻, leaving only 50 to 100 mEq to be excreted, thus resulting in the acidification of urine.

• Mechanism of H+ Secretion:
  • The secretion of H+ occurs through the formation of carbonic acid (H₂CO₃).
  • Carbon dioxide (CO₂), produced in tubular cells or derived from tubular fluid, combines with water (H₂O) to form carbonic acid in the presence of the enzyme carbonic anhydrase, which is abundantly present in the epithelial cells of renal tubules.
  • Carbonic acid immediately dissociates into H+ and HCO₃⁻:
  • 
    $ext{H}_2 ext{CO}_3 <br /> ightleftharpoons ext{H}^+ + ext{HCO}_3^-$
  • H+ is secreted into the lumen of:
  • Proximal convoluted tubule.
  • Distal convoluted tubule.
  • Collecting duct.

Specialized Cells for Acid-Base Handling

• Intercalated Cells (I Cells):
  • Located in the distal convoluted tubule and collecting duct, these specialized cells manage hydrogen and bicarbonate ions.

Mechanisms of H+ Secretion in the Renal Tubules

• Two Pumps Involved in H+ Secretion:

  1. Sodium-Hydrogen Antiport Pump:
  • When sodium ions (Na⁺) are reabsorbed from the tubular fluid into the tubular cells, H+ is secreted from the cells into the tubular fluid in exchange for Na⁺.
  • This process is mediated by the sodium-hydrogen antiport pump present in the tubular cells.

• Conclusion:

  • The kidney not only filters waste but also plays an essential role in regulating the body’s acid-base balance through intricate mechanisms involving the reabsorption of bicarbonate and secretion of hydrogen ions.