The cardiac sphincter, also known as the lower esophageal sphincter (LES), is a ring of muscle located at the junction of the esophagus and the stomach. Its primary function is to prevent the backflow of stomach contents, including acid, into the esophagus. This helps protect the esophagus from irritation and damage. When the cardiac sphincter is functioning properly, it opens to allow food to enter the stomach and closes to keep stomach acids contained. If the sphincter weakens or relaxes inappropriately, it can lead to conditions such as gastroesophageal reflux disease (GERD), causing symptoms like heartburn and discomfort.