Minerals of life – bicarbonate
Carbon dioxide (CO2), a gaseous waste product from cellular respiration, mixes with water to form carbonic acid (H2CO3). Carbonic acid is an intermediate specie in the production of bicarbonate. This reaction, without a catalyst, is usually very slow.
In the circulation, the reaction is catalyzed by carbonic anhydrase present in the red blood cells. This increases the rate of the reaction and causes the deprotonation of carbonic acid by the removal of one proton (H+) to form bicarbonate (HCO3-) in the blood plasma. In the blood, bicarbonate acts as a buffer to maintain the blood pH at 7.365.
This reaction is reversed in the lungs back to CO2 which is expelled as a gas. The solution of carbon dioxide (carbonic acid), otherwise known as respiratory acid or volatile acid, is the only acid that is excreted as a gas by the lungs.
These reactions, CO2 with water to form carbonic acid, deprotonation of carbonic acid to produce bicarbonate and the reversal reaction in the lungs to give CO2, is the mechanism by which CO2 is transported from the cells to the lungs where it is excreted. Not only that, the bicarbonate produced maintains the acid-base balance of the blood.
Apart from the bicarbonate that is produced in the blood stream from carbonic acid, bicarbonate is also produced in the pancreas as an aqueous buffer solution.
Bicarbonate is secreted from the pancreas after being stimulated by secretin. Secretin is produced by the S-cells of the duodenum, found in what is known as Crypts of Lieberkuhn. Smaller amounts are also produced in the jejenum of the small intestine.
Secretin is released by these cells in response to little amounts of acidic chyme from the stomach. Chyme is the name given to the stomach contents after digestion and churning of the food to break them into small particles.
Hydrochloric acid secreted by the cells in the stomach walls is added to the contents at this time to aid the breakdown. Secretin is secreted into the lumen of the intestines and the blood circulation. In the pancreas, it causes the release of the bicarbonate aqueous solution into the duodenum.
This bicarbonate solution, which is alkaline, neutralizes the acidic chyme. Apart from this, the bicarbonate solution creates a favourable pH for the intestinal digestive enzymes to function. One very important role of this bicarbonate ion, is that it prevents burns and irritation of the intestine that can lead to the development of duodenal ulcer.
Certain cells in the duodenum are gathered together to form what is known as Brunner’s gland. This gland also responds to secretin to release water and bicarbonate ions which also neutralize the acidic stomach chyme.
Bicarbonate is a part of the pH buffering system of the body. Together with carbonic acid, water, hydrogen ions and carbon dioxide, the buffer system provides quick resistance to severe pH changes both in the acid or base direction. By their buffering action, tissues that require strict pH balance for their functions are protected from damage and aided to function optimally. The tissues of the central nervous system, for example, which cannot stand extreme pH changes in whatever direction can be protected from damage.
Forms of bicarbonate
Bicarbonate can be joined to positive charge-carrying minerals to give the following forms: sodium bicarbonate (Baking soda), potassium bicarbonate, ammonium bicarbonate, magnesium bicarbonate, calcium bicarbonate and carbonic acid.
Therapeutic uses of bicarbonate
Bicarbonate is involved in the prevention and treatment of several disease conditions in the body. I will list a few of them here.
Bicarbonate is used in the treatment of metabolic acidosis (excess acid accumulation), it is also used to reduce the risk of metabolic acidosis in uncontrolled diabetes mellitus.
Also, acidosis that may occur in renal tubular disorders can be corrected by bicarbonate. In severe, long-standing diarrhoea, loss of bicarbonate may be as high as to require replacement. Treatment of upset stomach with hyperacidity can be achieved with bicarbonate.