The esophagus is a soft tube that is usually about 25 centimeters in length. When food or liquids are passed from the mouth, it is pushed down the esophagus by circular muscles. These muscles squeeze in top to bottom order, pushing the food and liquid down, like toothpaste being pushed through a tube. At the bottom of the esophagus is a barrier called a sphincter. This muscle is like a door that opens to let the esophagus contents enter the stomach. The sphincter then closes to keep stomach contents from coming up. Sometimes this muscle doesn't work well and stomach juices splash into the esophagus causing acid reflux and heartburn.