From the Research
H+ is secreted in the kidney primarily as combined forms rather than as free ions. The kidney uses several mechanisms to secrete H+ ions, including the Na+/H+ exchanger, H+-ATPase pumps, and H+/K+-ATPase pumps, but these protons quickly combine with buffer systems rather than existing as free ions in the tubular fluid. The main buffer systems in the kidney include bicarbonate, phosphate, and ammonia/ammonium. When H+ is secreted into the tubular lumen, it combines with bicarbonate to form carbonic acid, which then breaks down into water and carbon dioxide. H+ can also combine with phosphate buffers (HPO4²⁻) to form H₂PO₄⁻, or with ammonia (NH₃) to form ammonium (NH₄⁺) 1.
Some key points to consider:
- The kidney's ability to secrete H+ in combined forms allows it to eliminate acid from the body while maintaining a relatively safe pH in the tubular fluid, which is crucial for acid-base homeostasis and preventing conditions like metabolic acidosis 2.
- Metabolic acidosis can be of a kidney origin or an extrarenal cause, and the kidney plays a critical role in regulating acid-base balance by secreting H+ ions and reabsorbing bicarbonate 3.
- The distal nephron, including the cortical distal tubule, is a nephron segment responsible for a sizeable portion of bicarbonate reabsorption and titratable acid generation, as well as for bicarbonate secretion under appropriate metabolic conditions 4.
Overall, the kidney's ability to secrete H+ in combined forms is essential for maintaining acid-base homeostasis and preventing conditions like metabolic acidosis. This process is critical for eliminating acid from the body while maintaining a relatively safe pH in the tubular fluid.