Between the oesophagus and the stomach is a muscle valve called the lower oesophageal sphincter (LES). Its role is to open when you swallow and close afterwards to keep stomach acid where it belongs.
GERD develops when the LES becomes weak or relaxes inappropriately, allowing acid to flow back into the oesophagus. Over time, repeated exposure to acid irritates the oesophageal lining and causes symptoms.

