Lower Esophageal Sphincter
These are images of the end of the esophagus. There is a specialized muscle here which acts like a valve and which is called the lower esophageal sphincter (LES). It remains closed most of the time, only opening to allow swallowed food and liquid to be swept through into the stomach. When you belch, the air pressure in the stomach overcomes the pressure of the valve and the air you have swallowed bursts up the esophagus past this valve. The LES in Image 1 is closed while that in Image 2 is open.