becoz of d secretion of HCl by d oxyntic cells of gastric glands....which makes d pH acidic...nearly 1.5-2........dis pH is necessary for d action of pepsin...

Stomach acid, also known as gastric acid, is the primary digestive agent in the body. Dr. Ronen Arai, writing for YourTotalHealth.com, says stomach acid is designed "to help break down foods and aid digestion." Consequently, stomach acid is most effective in digestion when it has a low pH level, making it highly acidic. Incorrect pH and abnormal levels of stomach acid can lead to medical complications, including acid reflux, ulcers and gastritis (stomach inflammation).

Gastric acid is produced by parietal cells (also called oxyntic cells) in the stomach. Its secretion is a complex and relatively energetically expensive process. Parietal cells contain an extensive secretory network (called canaliculi) from which the gastric acid is secreted into the lumen of the stomach. These cells are part of epithelial fundic glands in the gastric mucosa. The pH of gastric acid is 2 to 3 in the human stomach lumen, the acidity being maintained by the proton pump H⁺/K⁺ ATPase. The parietal cell releases bicarbonate into the blood stream in the process, which causes a temporary rise of pH in the blood, known as alkaline tide.

The resulting highly acidic environment in the stomach lumen causes proteins from food to lose their characteristic folded structure (or denature). This exposes the protein's peptide bonds.The chief cells of the stomach secrete enzymes for protein breakdown (inactive pepsinogen and renin). Gastric acid activates pepsinogen into pepsin–this enzyme then helps digestion by breaking the bonds linking amino acids, a process known as proteolysis. In addition, many microorganisms have their growth inhibited by such an acidic environment which is helpful to prevent infection.

The stomach is acidic. In particular HCl, with a pH between about 2-4. The acidic environment is necessary for proper digestion. In addition, the low pH kills many pathogens that are ingested, thus protecting the body from some potential disease conditions.

The pH of the stomach should be acidic for the pepsin to act as it works in acidic conditions .