Final answer:
The opening of the gastroesophageal sphincter is triggered by the process of peristalsis in the esophagus, which propels the food bolus toward the stomach leading to the sphincter's relaxation.
Step-by-step explanation:
The event that triggers the opening of the gastroesophageal sphincter is peristalsis in the esophagus. Once the food bolus travels down the esophagus and nears the stomach, the pressure exerted by the bolus and the distention of the esophagus trigger a short reflex that causes the lower esophageal sphincter to relax. This relaxation allows the bolus to pass into the stomach. The primary function of this sphincter is to open for the passage of food into the stomach and then close to prevent the stomach contents from refluxing back into the esophagus, which could lead to conditions like heartburn or gastroesophageal reflux disease (GERD).