Gastric acid
The main constituent of gastric acid is hydrochloric acid which is produced by parietal cells (also called oxyntic cells) in the gastric glands in the stomach. Its secretion is a complex and relatively energetically expensive process. Parietal cells contain an extensive secretory network (called canaliculi) from which the hydrochloric acid is secreted into the lumen of the stomach. The pH of gastric acid is 1.5 to 3.5 in the human stomach lumen, the acidity being maintained by the proton pump H/K ATPase. The parietal cell releases bicarbonate into the bloodstream in the process, which causes a temporary rise of pH in the blood, known as an alkaline tide.
Metabolic acidosis
Although blood gas sampling is not always essential for the diagnosis of acidosis, a low pH (in either a venous or arterial sample) does support the diagnosis. If the pH is low (under 7.35) and the bicarbonate levels are decreased (<24 mmol/L), metabolic acidemia is present, and metabolic acidosis is presumed. If the patient has other coexisting acid-base disorders, the pH may be low, normal or high in the setting of metabolic acidosis. If a setting of a cause for metabolic acidosis being noted in the patient's history, a low serum bicarbonate indicates metabolic acidosis even without measurement of serum pH.