Final answer:
Gastrin is a hormone secreted by G cells in the stomach that stimulates parietal cells to release hydrochloric acid, aiding protein digestion. It is regulated by neural and hormonal mechanisms and can affect other organs like the pancreas, liver, and gallbladder.
Step-by-step explanation:
The hormone gastrin plays a critical role in the digestive process. It is secreted by G cells in the mucosal lining of the stomach in response to the presence of food, especially proteins. Upon stimulation, gastrin targets the parietal cells of the stomach lining, prompting them to secrete gastric acid (hydrochloric acid or HCl). This increase in stomach acidity helps break down proteins, making them easier to digest.
Gastrin secretion occurs during the gastric phase of digestion and is regulated by the nervous system and other hormones. However, the hormone somatostatin can inhibit gastrin's action and decrease stomach acidity once the stomach becomes empty, as part of a negative feedback mechanism. Gastrin and other digestive hormones like secretin and cholecystokinin also affect other organs such as the pancreas, liver, and gallbladder by triggering the release of digestive juices and enzymes.