Stomach acid.
http://en.wikipedia.org/wiki/Stomach_acid Gastric acid is a digestive fluid, formed in the
stomach. It has a
pH of 1.5 to 3.5 and is composed of
hydrochloric acid (HCl) (around 0.5%, or 5000
parts per million), and large quantities of
potassium chloride (KCl) and
sodium chloride (NaCl). The acid plays a key role in digestion of
proteins, by activating
digestive enzymes, and making ingested proteins unravel so that digestive enzymes can break down the long chains of
amino acids.
Gastric acid is produced by cells lining the stomach, which are coupled to systems to increase acid production when needed. Other cells in the stomach produce
bicarbonate to
buffer the acid, ensuring the pH does not drop too low (acid reduces pH). Also cells in the beginning of the small intestine, or
duodenum, produce large amounts of bicarbonate to completely neutralize any gastric acid that passes further down into the digestive tract. The bicarbonate-secreting cells in the stomach also produce and secrete
mucus. Mucus forms a viscous physical barrier to prevent gastric acid from damaging the stomach.