bicarbonate

Bicarbonate is the blood's main chemical sponge for soaking up excess acid. When acid appears in the body, bicarbonate combines with it to blunt the change in pH, acting as the body's most important buffer base.

It works as one half of a partnership with carbon dioxide. Carbon dioxide and water can turn into acid and bicarbonate, and back again; the lungs control the carbon dioxide side within minutes, while the kidneys control the bicarbonate side over hours to days by reclaiming or excreting it in the urine. Together they hold blood pH near 7.4.

Because of this teamwork, bicarbonate is a key clue in reading an arterial blood gas. A primary fall in bicarbonate signals a metabolic acidosis, while a rise signals a metabolic alkalosis; a change in bicarbonate that moves in the same direction as carbon dioxide usually reflects the kidney compensating for a breathing problem rather than a separate disease.