The kidney can correct any imbalances by:
- Removing excess acid (hydrogen ion) or bases (bicarbonate) in the urine and
- Restoring the bicarbonate concentration in the blood to normal
The kidney cells produce a constant amount of hydrogen ion and bicarbonate because of their own cellular metabolism (production of carbon dioxide). Through a carbonic anhydrase reaction similar to the red blood cells, hydrogen ions get produced and secreted into the lumen of the nephron. Also, bicarbonate ions get produced and secreted into the blood. In the lumen of the nephron, filtered bicarbonate combines with secreted hydrogen ions to form carbon dioxide and water (carbonic anhydrase is also present on the luminal surface of the kidney cells). Whether the kidney removes hydrogen ions or bicarbonate ions in the urine depends upon the amount of bicarbonate filtered in the glomerulus from the blood relative to the amount of hydrogen ions secreted by the kidney cells. If the amount of filtered bicarbonate is greater than the amount of secreted hydrogen ions, then bicarbonate will be lost in the urine. Likewise, If the amount of secreted hydrogen ion is greater than the amount of filtered bicarbonate, then hydrogen ions will be lost in the urine (i.e. acidic urine).
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