
Your blood maintains a constant concentration of hydrogen
ion (pH) by a chemical mixture of hydrogen ions and sodium bicarbonate.
The sodium bicarbonate is produced by the carbon dioxide (CO
2) formed in the cells as a byproduct of many chemical reactions.
The CO2 enters the blood in the capillaries,
where red blood cells contain an enzyme called carbonic anhydrase that
helps combine CO 2 and water (H
2O) to form carbonic acid (H 2 CO3
) quickly. The carbonic acid formed then rapidly separates
into hydrogen ions (H+ ) and bicarbonate ions
(HCO3-).
This reaction can also proceed in the reverse direction, whereby sodium bicarbonate
plus hydrogen ion yields carbon dioxide and water.
Carbonic
Anhydrase
CO 2 + H 2 O
<---------> H 2 CO3
<---------> H+ + HCO
3 -
The correct pH is maintained by keeping
the ratio of hydrogen ion to bicarbonate in the blood constant. If you add acid
(hydrogen ion) to the blood, then you will reduce the bicarbonate concentration
and alter the pH of the blood. Similarly, if you reduce the hydrogen ion by
adding alkali, you will increase the bicarbonate concentration and alter the
pH of the blood.
Now, the acid/base balance of our
blood changes in response to many things including:
- Diet - diets rich in meats provide acids
to the bloods when digested. In contrast, diets rich in fruits and vegetables
make our blood alkaline because they are rich in bicarbonates.
- Exercise - exercising muscles produce
lactic acid that must be eliminated from the body or metabolized.
- Breathing - high altitude causes rapid
breathing that makes our blood alkaline. In contrast, certain lung diseases
that block the diffusion of oxygen can cause the blood to be acidic.
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.