The kidneys play a crucial role in maintaining the body's acid-base balance, a vital aspect of homeostasis, by regulating the pH of the blood.
The normal pH range of blood is tightly regulated between 7.35 and 7.45, and the kidneys contribute to maintaining this balance by excreting acid or base and reabsorbing bicarbonate (HCO3^-) from the urine.
Excretion of Hydrogen Ions
The kidneys help maintain acid-base balance primarily through the excretion of hydrogen ions (H+) in the urine. Nephrons, the functional units of the kidney, contain cells that can secrete H+ into the tubular fluid.
This process is essential for removing excess acid from the body.
The excretion of hydrogen ions is facilitated through several mechanisms:
1. Ammonia Buffering:
Process: Ammonia (NH3) binds with H+ to form ammonium (NH4+), which is excreted in urine.
2. Phosphate Buffering:
Process: Dihydrogen phosphate (H2PO4-) binds with H+ to form monohydrogen phosphate (HPO4^2-), which is excreted in urine.
Reabsorption and Production of Bicarbonate
1. Reabsorption:
Process: Most filtered bicarbonate is reabsorbed in the proximal tubule, helping maintain the body's base balance.
2. Production:
Process: Kidneys generate new bicarbonate through glutamine metabolism and the excretion of ammonium (NH4+) and titratable acids, which adds new bicarbonate to the blood.
Compensation for Acid-Base Imbalances
The kidneys respond to and compensate for imbalances in the body's acid-base status:
1. Metabolic Acidosis:
Response: The kidneys increase H+ excretion and bicarbonate reabsorption to raise blood pH.
2. Metabolic Alkalosis:
Response: The kidneys reduce H+ excretion and bicarbonate reabsorption to lower blood pH.