The cardiac sphincter is the valve which closes off the end of the eusophagus to the beginning of the stomach (the cardia). This sphincter is so strong and folds over so much in horses and rabbits that they are unable to regurgitate or vomit. Humans who have heartburn or acid reflux have a weak cardiac sphincter, allowing HCl (hydrochloric acid) from their stomach to get back up into their eusophagus, causing pain. Enough of this can cause eusophogeal ulceration.


In this cartoon picture the cardiac sphincter is labeled number 4 (end of eusophagus/ beginning of stomach)