Final answer:
Acid entering the small intestine stimulates the secretion of gastric juice, which helps in digestion.
Step-by-step explanation:
When acid enters the small intestine, it stimulates the release of gastric juice. The stimulation occurs through various mechanisms, including the activation of stretch receptors and the release of hormones such as acetylcholine and gastrin. These stimuli trigger cells in the stomach wall to increase their secretion of digestive juices, preparing the digestive system for the incoming food. Overall, the presence of acid in the small intestine promotes the secretion of gastric juice to aid in the digestion process.