Bicarbonate (HCO3−) is the primary buffer in the blood and plays a central role in maintaining the body's acid-base balance. It is produced by the kidneys and serves as the main defence against metabolic acid accumulation. The kidneys regulate bicarbonate levels by reabsorbing it from the renal tubules or excreting excess into the urine.

Low bicarbonate (metabolic acidosis) can indicate kidney disease, diabetic ketoacidosis, severe diarrhoea, or toxic ingestion. High bicarbonate (metabolic alkalosis) typically occurs with prolonged vomiting, diuretic use, or hypokalaemia. Bicarbonate is one of the key electrolytes measured in a urea and electrolytes (U&E) panel and is routinely used to assess kidney function and overall metabolic status.