There are muscles (esophageal sphincters) at both ends of the tube that carries food to your stomach (the esophagus). These muscles relax to let food pass. Then they tighten to keep stomach acid down. When the lower esophageal sphincter (LES) doesn't tighten enough, acid can flow back (reflux) from your stomach into your esophagus. This may cause heartburn. In some cases, the upper esophageal sphincter (UES) also doesn't work well. Then acid can travel higher and enter your throat (pharynx). In many cases, this causes throat symptoms.