Gastrin is produced by special cells (called G cells) in the stomach, duodenum and pancreas, in response to physical stimulation due to the presence of food.
it stimulates the production of gastric juice by the parietal cells in the gastric glands. Gastric juice contains a mixture of water, hydrochloric acid and other inorganic ions, enzymes (pepsin, rennin), mucus and intrinsic factor.
Gastric juice changes the pH of the food content from 6.7 to 2, providing acidic conditions that will enhance digestion. When there is sufficient gastric juice present (around 1 to 1.5 litres), the production of gastrin will stop.