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.
Rumen
Microbes in the reticulorumen eventually flow out into the omasum and the remainder of the alimentary canal. Under normal fermentation conditions the environment in the reticulorumen is weakly acidic and is populated by microbes that are adapted to a pH between roughly 5.5 and 6.5; since the abomasum is strongly acidic (pH 2 to 4), it acts as a barrier that largely kills reticulorumen flora and fauna as they flow into it. Subsequently, microbial biomass is digested in the small intestine and smaller molecules (mainly amino acids) are absorbed and transported in the portal vein to the liver. The digestion of these microbes in the small intestine is a major source of nutrition, as microbes usually supply some 60 to 90% of the total amount of amino acids absorbed. On starch-poor diets, they also provide the predominant source of glucose absorbed from the small intestinal contents. Under conditions of ruminal acidosis, when the environment of the reticulorumen has become too acidic (usually due to excessive fermentation of starches and sugars into VFA and lactate), microbes that favor a lower pH may start to dominate the ecosystem of the reticulorumen. This gives rise to rumen acidosis and often feed intake of the ruminant will drop.