The pH of the stomach is always on the acidic side of the pH scale, but it varies from a pH below 3.0 to a pH between 5.0 and 6.0. Stomach pH is lowest when food is being digested and highest when the stomach is empty.
The reason the stomach is acidic is because its lining secretes gastric juice, which contains hydrochloric acid (HCl). While gastric juice is secreted with a pH as low as 0.8, it is immediately diluted in the stomach lumen and its pH rises to between 1.5 and 3.5. Sodium bicarbonate neutralizes the acid, so stomach pH rises to 5.0 to 6.0 when the stomach is empty.
It’s a common misconception the acid in gastric juice digests proteins. The low pH causes proteins to unfold, exposing their peptide bonds. Acidity also activates the enzyme pepsinogen, which becomes pepsin and breaks the peptide bonds, digesting the proteins into amino acids.
How Is Stomach pH Measured?
Medical professionals use the stomach acid test to measure the pH of the acid in the stomach and the pH of stomach contents. Typically, this test is done following a period of fasting so the stomach only contains fluid and not food. Stomach fluid is removed via a tube inserted through the mouth and esophagus into the stomach and then its pH is measured. In another variation of the test, the hormone gastrin is injected and the stomach’s ability to produce acid is measured.
Chemical Composition of Gastric Juice
Gastric juice isn’t pure hydrochloric acid. It is a complex mixture of water, acid, enzymes, and mucous. The components of gastric juice are:
- Water – Water doesn’t affect the pH of the stomach, but it does serve to provide enough liquidity that food, enzymes, and acids can readily mix together. Some enzymes require water in order to function.
- Mucous – Cells in the mouth, esophagus, and stomach produce mucus. It eases the passage of food through the gastrointestinal tract and protects the lining of the stomach from being attacked by acid. Neck cells also secrete bicarbonate, which buffers the acid and controls pH.
- Hydrochloric Acid – This potent acid is secreted by the parietal cells of the stomach. It kills bacteria and other potential pathogens in food and converts the enzyme pepsinogen into pepsin, which breaks secondary and tertiary proteins into smaller, more easily digested molecules.
- Pepsinogen – Pepsinogen is secreted by chief cells in the stomach. Once it’s activated by low pH, it helps digest proteins.
- Hormones and Electrolytes – Gastric juice also contains hormones and electrolytes, which aid in organ function, food digestion, and nutrient absorption. The enteroendocrine cells secrete multiple hormones.
- Gastric Lipase – Lipase is an enzyme made by chief cells in the stomach that aids in breaking up short-chain and medium-chain fats.
- Intrinsic Factor – The parietal cells of the stomach secrete intrinsic factor, which is necessary for vitamin B-12 absorption.
- Amylase – Amylase is an enzyme found primarily in saliva, where it acts to break down carbohydrates. It’s found in the stomach because you swallow saliva as well as food, but it is inactivated by the low pH. Additional amylase is secreted into the small intestine.
The churning of the stomach mixes food and gastric juice to produce chyme. Chyme exits the stomach and enters the small intestine. Here, the remaining acid is neutralized, digestion continues, and nutrients are absorbed. Water and electrolytes are recovered in large intestine, plus microorganisms release nutrients from fermentation.
Why Gastric Juice Doesn’t Digest the Stomach
The acid and enzymes in gastric juice could digest the stomach, which is mostly protein. So, why doesn’t this happen? The reason the stomach doesn’t digest itself is because it has a lining of epithelial cells that secrete protective mucus. The mucus barrier releases the alkaline bicarbonate anion, which neutralizes acid to produce water and carbon dioxide. When acid is neutralized, the pH near the stomach lining rises outside the optimal range of gastric enzymes and prevents them from dissolving the tissue. However, there are disorders which increase gastric acid production beyond the neutralizing capacity of the mucosal layer. When too much acid is produced, a gastric ulcer can occur.
References
- Guyton, Arthur C.; John E. Hall (2006). Textbook of Medical Physiology (11 ed.). Philadelphia: Elsevier Saunders. ISBN 0-7216-0240-1.
- Loomis, Howard F. “Digestion in the Stomach.” Food Enzyme Institute.
- Marieb, Elaine N.; Hoehn, Katja; Hoehn, Katja N. (2018). Human Anatomy and Physiology (11th ed.) Pearson Education, Inc. ISBN 0134580990.
- Medline Plus. “Stomach Acid Test.” U.S. National Library of Medicine.