Renal Acid-Base Control
- The kidneys exert control over the extracellular fluid pH by regulating excretion of hydrogen ions and bicarbonate as well as displaying the capacity to generate novel ECF bicarbonate. When ECF pH is excessively low, the kidneys resorb all of the filtered bicarbonate, actively secrete hydrogen ions, and generate novel ECF bicarbonate, thus slowly raising ECF pH. When ECF pH is excessively high, the kidneys can excrete large amounts of filtered bicarbonate and reduce hydrogen ion secretion, thus slowly reducing ECF pH. Because of their capacity to definitively eliminate acids without consumption of physiological buffers and their ability to generate novel bicarbonate, the kidneys are the organ which controls long-term acid-base.
- The kidneys perform a number of key processes responsible for long-term acid-base control including direct secretion of H+, novel generation of bicarbonate, excretion of bicarbonate, and excretion of fixed acids. Given the multiple processes which the kidneys have at their disposal to modulate ECF pH, we provide an integrated view of their function in Renal Response to Acid-Base Imbalance.
- H+ Secretion and Novel Bicarbonate Generation
- The kidneys can directly influence the extracellular pH by eliminating ECF hydrogen ions through their urinary excretion. Importantly, excretion of hydrogen ions by the kidneys is molecularly coupled to novel generation of bicarbonate which is subsequently added to the extracellular fluid, thus replenising the ECF bicarbonate buffer. The specific molecular mechanisms and regulation of these processes are covered in Renal Acid Excretion.
- Bicarbonate Excretion
- The bicarbonate buffer is the principal physiological buffer of the extracellular fluid. As discussed in bicarbonate buffer, the extracellular pH is largely determined by the ratio of the Weak Acid (CO2) to Weak Base (HCO3-) form of this buffer. The kidneys can influence the extracellular pH by regulating urinary excretion of bicarbonate HCO3- as discussed in renal bicarbonate excretion. It should also be pointed out that as mentioned above, the kidneys can also synthesize and add novel bicarbonate to the ECF as part of renal acid excretion.
- Fixed Acid Elimination
- As discussed in physiological acid production, normal and pathological metabolic processes can generate a number of strong acids which are added to the extracellular fluid. Although these acids immediately release a free hydrogen ion which can be eliminated by other processes, the remaining molecule must also be eliminated to prevent its gradual build up in the extracellular fluid. The only organ which can ultimately eliminate these fixed acids is the kidney which does so through their urinary excretion.