Final answer:
A. gastroesophageal sphincter The bolus is transferred into the stomach through the gastroesophageal sphincter during deglutition, which prevents stomach contents from re-entering the esophagus when not swallowing.
Step-by-step explanation:
During deglutition, the bolus passes into the stomach from the esophagus through the gastroesophageal sphincter. The gastroesophageal sphincter, also known as the lower esophageal sphincter, opens in response to the approaching bolus, allowing it to enter the stomach from the esophagus. When not actively swallowing, this sphincter remains contracted to prevent the contents of the stomach, including acid, from traveling back up into the esophagus, a condition known as acid reflux or "heartburn." The action of the sphincter is part of a coordinated process involving peristalsis that propels food down the esophagus.
The bolus passes into the stomach from the esophagus through the gastroesophageal sphincter. This sphincter is located at the stomach end of the esophagus. During swallowing, the pressure exerted by the bolus of food causes the gastroesophageal sphincter to open, allowing the bolus to enter the stomach. When there is no swallowing action, the sphincter closes to prevent the contents of the stomach from traveling back up the esophagus.