The cardia!

The cardia (or esophagogastric junction or gastroesophageal junction) is the anatomical term for the junction orifice of the stomach and the esophagus. At the cardia, the mucosa of the esophagus transitions into gastric mucosa.
The cardia is also the site of the lower esophageal sphincter (also termed cardiac sphincter, gastroesophageal sphincter, and esophageal sphincter


The stomach generates strong acids and enzymes to aid in food digestion. This digestive mixture is called gastric juice. The inner lining of the stomach has several mechanisms to resist the effect of gastric juice on itself, but the mucosa of the esophagus does not. The esophagus is normally protected from these acids by a one-way valve mechanism at its junction with the stomach. This one-way valve is called the lower esophageal sphincter (LES), and prevents gastric juice from flowing back into the esophagus.
During peristalsis, the LES allows the food bolus to pass into the stomach. It prevents chyme, a mixture of bolus, stomach acid, and digestive enzymes, from returning up the esophagus. The LES is aided in the task of keeping the flow of materials in one direction by the diaphragm.

chris