The cardiac sphincter is a ring of smooth muscle fibers at the junction of the esophagus and stomach. It is also called gastroesophageal sphincter or the lower esophageal sphincter (LES). A sphincter is a specialized type of muscle with an opening that relaxes and contracts, allowing this muscular valve to open and close as necessary. The valve is in a closed position the majority of the time, and it opens only when necessary in order to allow food and liquid to pass into the stomach. The cardiac sphincter closes to allow the food to stay within the stomach so it can be digested. Cardiac sphincter, working with the pyloric sphincter keeps the stomach content from moving elsewhere. Air pressure in the stomach can sometimes overcome this muscle. It causes the person to belch, forcing the valve open in order to allow the excess air to be released. The function of the cardiac sphincter is to prevent a back flow of materials back into the esophagus. When this valve does not function properly, a variety of medical issues like acid reflux or GERD, may occur.