The quality of being acid describes a solution with a pH less than 7.0. The contents of the stomach or abomasum are normally acid because of the secretion of 0.15 M hydrochloric acid by the parietal cells in the gastric mucosa. This acid is bacteriocidal for many ingested organisms; it also provides the necessary pH for the conversion of pepsinogen to pepsin and for the latter to start the digestion of dietary protein. The gastric mucosa is protected from self-digestion by an unstirred layer of mucus, made alkaline with bicarbonate.
Because of its high content of bicarbonate, the pancreatic juice secreted into the duodenum is alkaline, e.g. pH 8.0. In addition, bile and intestinal juice both tend to be alkaline and so these three secretions soon neutralize the gastric contents entering the duodenum and raise the pH of the duodenal contents to 6.0–7.0. By the time the chyme reaches the jejunum, its reaction is neutral or may become alkaline, depending on the species. This has an important bearing on the solubility of calcium phosphate and the absorption of calcium ions from the upper part of the small intestine (see Hyperparathyroidism).
The pH of the contents of the large intestine is close to neutrality; however, in the horse, and other species in which there is a good deal of cellulose fermentation in the caecum and colon with the production of volatile fatty acids, the pH of the gut contents in these regions is nearer 6.0 than 7.0.
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