Final answer:
The lower oesophagal sphincter remains constricted after a bolus enters the stomach to protect the oesophagus from gastric juices and prevent acid reflux.
Step-by-step explanation:
After a bolus has entered the stomach, the oesophagus is protected from gastric juice by continued constriction of the lower oesophagal sphincter. This sphincter acts as a muscular valve located at the stomach end of the oesophagus, which remains closed most of the time, except during swallowing when it relaxes briefly to allow food to pass through. The lower oesophagal sphincter is crucial in preventing the backflow of stomach contents, including acidic gastric juices, which can lead to conditions such as acid reflux or gastroesophageal reflux disease (GERD).