Gastric juice is a highly acidic fluid produced by glands embedded in the stomach lining, representing a mixture of potent chemicals and enzymes. This digestive secretion is released into the stomach cavity to begin the process of breaking down food. The fluid’s low pH is necessary for initiating chemical digestion and offering a defense against ingested microbes. The approximately 1.2 to 1.5 liters of gastric juice secreted daily transforms the food mass into a semi-liquid substance called chyme, preparing it for the next phases of digestion in the small intestine.
The Components of Gastric Juice
Gastric juice is formed by specialized cells within the stomach’s mucosal layer, each contributing a specific compound. Hydrochloric acid (HCl) is a major component, produced by parietal cells, which drives the fluid’s extreme acidity, typically ranging from pH 1.5 to 3.5. These same parietal cells also secrete intrinsic factor, a glycoprotein with a function separate from digestion.
Chief cells secrete pepsinogen, an inactive precursor to a protein-digesting enzyme. The stomach lining also secretes a thick, alkaline mucus, produced by mucous neck cells and surface epithelial cells. These secretions—hydrochloric acid, pepsinogen, intrinsic factor, and mucus—mix with water and electrolytes to form gastric juice.
Chemical Digestion and Sterilization
Hydrochloric acid’s extreme acidity is foundational to two primary functions: chemical breakdown of food and sterilization. When food enters the stomach, the low pH immediately causes proteins to denature, meaning their complex structures unfold. This unfolding exposes internal peptide bonds, making the protein molecules more accessible to subsequent digestive enzymes.
The acid also converts inactive pepsinogen, secreted by the chief cells, into its active form, the enzyme pepsin. Pepsin is a protease that begins breaking the denatured proteins into smaller fragments called polypeptides. This initial breakdown reduces large food particles into more manageable pieces for subsequent enzyme action further down the digestive tract.
The harsh, acidic environment also serves as a crucial line of defense. Stomach acid kills most bacteria, viruses, and other pathogens ingested with food and drink. This sterilizing effect reduces the microbial load entering the gastrointestinal system, functioning as a protective barrier against infections.
Essential Protective and Absorptive Roles
The stomach is protected from digesting itself by a thick layer of mucus that covers the epithelial cells of the stomach wall. This mucus is rich in bicarbonate, a basic compound that neutralizes the acid directly adjacent to the stomach lining. This gel-like barrier prevents the corrosive hydrochloric acid and the enzyme pepsin from damaging the stomach’s own tissues.
Gastric juice also contains intrinsic factor, a protein released by the parietal cells along with the acid. Intrinsic factor does not participate in digestion but is required for the absorption of Vitamin B12 (cobalamin). Stomach acid frees B12 from food proteins, allowing it to bind to the intrinsic factor. This complex is then carried to the small intestine for absorption into the bloodstream.