Regulating Blood Composition

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).

Let's consider a few examples:

Acid Diet Hydrogen ions added to the blood by breaking down a meat-rich diet combine with bicarbonate in the blood and form carbon dioxide and water. This reaction reduces the bicarbonate concentration and the pH in the blood. The decreased bicarbonate concentration in the blood reduces the amount of bicarbonate filtered in the glomerulus. All of the filtered bicarbonate combines with the hydrogen ion secreted by the kidney cells in the lumen to form carbon dioxide and water. Because the filtered load of bicarbonate was less than the amount of hydrogen ion secreted by the kidney cells, there is an excess of hydrogen ion in the urine. The amount of bicarbonate secreted from the kidney cells into the blood was equal to the hydrogen ion secreted into the lumen and greater than the filtered load of bicarbonate from the blood -- therefore, the blood has a net gain of bicarbonate. This process continues to lose hydrogen ions in the urine and gain bicarbonate in the blood until the concentrations of hydrogen (pH) and bicarbonate ions in the blood are restored to normal.
Alkaline Diet Bicarbonate added to the blood from the fruit or vegetable-rich diet combines with hydrogen ions to form carbon dioxide and water. This reaction reduces the hydrogen ion concentration and increases the pH. The increased bicarbonate concentration increases the amount of bicarbonate filtered in the glomerulus. The filtered bicarbonate exceeds the amount of hydrogen ion secreted by the kidney cell, and excess bicarbonate is lost in the urine. The amount of bicarbonate secreted from the kidney cells into the blood was equal to the hydrogen ions secreted into the lumen and less than the filtered load of bicarbonate from the blood -- therefore, the blood has a net loss of bicarbonate. This process continues to lose bicarbonate in the urine and reduce the bicarbonate in the blood until the concentrations of hydrogen (pH) and bicarbonate ions in the blood are restored to normal.

Now that we have seen how the kidneys regulate the composition of our blood, let's look at how they help regulate our blood pressure.