Gastric juice is secreted by the gastric glands, present in the gastric mucosa of fundus and body of the stomach. The glands are long coiled tubular structures, which arise from the gastric pit, from the surface mucosa. The gland has three types of cells, namely neck cells, chief cells and parietal cells. The neck cell and the surface epithelial cells secrete mucus. The chief cells contain zymogen granules and secrete enzymes. The parietal cells or oxyntic cells are the source of HCl and intrinsic factor.
The volume of secretion per day is around 2 lit and the pH varies from 1 to 2. The low pH is due to the secretion of concentrated HCl. The inorganic constituents include, Na+, K+, Cl–, PO4 – and SO4 –. The organic substances present in the secretion are digestive enzymes, pepsinogen, rennin, lipase, mucin and intrinsic factor. The concentrated HCl in the gastric juice, is necessary to activate pepsinogen to pepsin and the optimum pH that is needed for this action will be 2. The extremely acidic pH acts as bactericidal and also it converts cane sugar to monosaccharides. The acidic pH in the upper part of duodenum facilitates iron absorption.
The beginning of protein digestion takes place in the stomach. Pepsin acts on protein and converts it to peptones. The enzyme is secreted from the chief cells as inactive pepsinogen. Gastric rennin is a milk curdling enzyme which is absent in humans. It is present in calves. The gastric lipase is a weak fat splitting enzyme.
Intrinsic factor is a glycoprotein and secreted from parietal cells of the body and the fundus of stomach. Intrinsic factor is one of the important constituents of gastric juice which is required for the absorption of vit B12 (extrinsic factor). The absorption of vit B12 occurs in the terminal ileum.
Mucus in the gastric juice comes from two sources. Besides the surface epithelium of the gastric mucosa, the neck cells of the gastric glands also secrete mucus. The surface epithelium of the mucosa in addition to mucus, release HCO3 –. The mucus and the bicarbonate form a gel in the lining of gastric mucosa. The presence of this gel and the mucus from neck cells, protect the gastric mucosa from the action of acid. Infact, there is a pH gradient from the lumen to the mucosal wall of the stomach. The pH in the mucosa is 7, while in the lumen, it is between 1 and 2. The presence of bicarbonate and mucus forms the acid mucosal barrier. In certain conditions like chronic stress, and substances like alcohol, vinegar, aspirin and other nonsteroidal anti inflammatory drugs(NSAID) have a tendency to erode the acid mucosal barrier. Loss of this barrier leads to gastric ulcer.

Source: Textbook of Physiology, 3E (Chandramouli) (2010)