What stops the food from coming back up?

The primary mechanism that prevents food from coming back up is the lower esophageal sphincter (LES), a ring of muscle at the junction of the esophagus and stomach. The LES remains tightly closed to keep stomach contents from refluxing into the esophagus. Additionally, the stomach's acidic environment and peristaltic movements help process food and prevent regurgitation. Proper functioning of these mechanisms is crucial for normal digestion and preventing conditions like gastroesophageal reflux disease (GERD).