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.
Here's how the kidneys manage this complex process:
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:
Ammonia (NH3) is produced by the cells in the proximal tubule and acts as a buffer by combining with H+ to form ammonium (NH4+), which is then excreted in the urine.
2. Phosphate Buffering:
Dihydrogen phosphate (H2PO4^-) in the tubular fluid also acts as a buffer by accepting H+ and forming monohydrogen phosphate (HPO4^2-), which is excreted in the urine.
Reabsorption and Production of Bicarbonate
Bicarbonate (HCO3^-) is a critical buffer in the blood that neutralizes excess acid. The kidneys regulate bicarbonate in two ways:
1. Reabsorption:
Most of the filtered bicarbonate is reabsorbed by the nephrons, primarily in the proximal tubule.
This process is crucial for conserving the body's base and is closely linked to the excretion of H+.
For every H+ ion that is secreted into the tubular fluid and bound by a buffer, one bicarbonate ion is reabsorbed into the bloodstream.
2. Production:
The kidneys also generate new bicarbonate ions, which are added to the blood.
This occurs through the metabolism of glutamine in the renal cells and the excretion of ammonium (NH4+) and titratable acids in the urine, which correspond to the addition of new bicarbonate ions to the bloodstream.
Compensation for Acid-Base Imbalances
The kidneys respond to and compensate for imbalances in the body's acid-base status:
1. Metabolic Acidosis:
In response to a decrease in blood pH (acidosis), the kidneys increase the excretion of H+ and enhance the reabsorption and production of bicarbonate, helping to raise the blood pH back to normal.
2. Metabolic Alkalosis:
Conversely, if the blood pH is too high (alkalosis), the kidneys can decrease the excretion of H+ and reduce the reabsorption of bicarbonate, helping to lower the blood pH to within the normal range.