Gastric acid
The highest concentration that gastric acid reaches in the stomach is 160 mM in the canaliculi. This is about 3 million times that of arterial blood, but almost exactly isotonic with other bodily fluids. The lowest pH of the secreted acid is 0.8, but the acid is diluted in the stomach lumen to a pH between 1 and 3.
Proton pump
Humans (and probably other mammals) have a gastric hydrogen potassium ATPase or H/K ATPase that also belongs to the P-type ATPase family. This enzyme functions as the proton pump of the stomach, primarily responsible for the acidification of the stomach contents (see gastric acid).