What sphincter is involved in Acid reflux disease?

The lower esophageal sphincter (LES) is the sphincter involved in acid reflux disease. The LES is a muscular valve that separates the stomach from the esophagus, allowing food to pass into the stomach but preventing the reflux of stomach contents back into the esophagus. When the LES relaxes inappropriately or becomes weak, it can allow stomach acid and other contents to flow back into the esophagus, causing symptoms of acid reflux disease such as heartburn and regurgitation.