BicarbonateHCO3 · Total CO2
The blood's main buffer against acid, and a clue to breathing and kidney function.
Bicarbonate is the body's principal chemical buffer: it mops up acid and keeps blood pH inside the narrow range enzymes need. The kidneys regulate how much is retained, and the lungs regulate the carbon dioxide it exists in balance with.
It matters because it moves when that balance is disturbed. It falls when acid is accumulating or being lost through the kidneys, and rises with prolonged vomiting or when carbon dioxide is being retained.
In practice it is read with chloride and sodium as part of the acid picture, and a formal answer comes from a blood gas test rather than this figure alone. Note that it is sometimes labelled total CO2 on a report, which is the same measurement.