Gastro-oesophageal reflux disease or GERD is a common condition where acid leaks back into esophagus from the stomach. When we eat food passes from the esophagus, which is the tube that carries food from mouth to stomach. At the end of esophagus, there is a muscle valve called lower oesophageal sphincter (LES) that prevents reflux (or backing up) of acid.

This sphincter relaxes during swallowing to allow food to pass. It then tightens to prevent flow in the opposite direction. But in GERD, sphincter relaxes between swallowing and the stomach contents and acid leak back to the esophagus.

Stomach has a tough lining that prevents the damage from acid, but esophagus does not. Thus the leak or the reflux irritates and causes damage to the esophagus, causing symptoms.