Heartburn does not actually involve the heart; rather, it’s a burning sensation in the chest caused by stomach acid that has traveled in the wrong direction due to a weakened or relaxed lower esophageal sphincter. When functioning properly, the sphincter acts like a gate; it relaxes to ensure the correct flow of food down into the stomach, and then it retightens to block the highly acidic stomach contents from coming back up and wreaking havoc. If heartburn becomes a frequent occurrence, it may be considered gastroesophageal reflux disease, or GERD.