Final answer:
The release of the hormone gastrin during digestion is caused by the presence of food in the stomach, initiating the gastric phase and stimulating the secretion of gastric acid, which is essential for protein digestion. Option a.
Step-by-step explanation:
In the process of digestion, the release of the hormone gastrin is primarily caused by the presence of food in the stomach option a. Gastrin plays a crucial role by stimulating the secretion of gastric acid by the parietal cells of the stomach mucosa, which is essential for the gastric phase of digestion. This process is crucial for protein digestion in the stomach, as gastrin also leads to the conversion of pepsinogen to pepsin, an enzyme necessary for breaking down proteins.
The secretion of gastrin stimulates not only the gastric acid needed for digestion but also induces muscle contractions that help in churning the stomach contents. Moreover, during the gastric phase, which begins when food reaches the stomach, the acidic environment necessary for the food processing is created and maintained by the action of gastrin, until negative feedback mechanisms, such as the hormone somatostatin, inhibit further secretion of hydrochloric acid to prevent excess acidity.