You may also have diarrhea or a fever. It is sometimes called stomach flu, but it is not the flu. Germs like viruses and bacteria can cause it. You can catch it from someone else who has it, or you can get it from food poisoning. Food poisoning can happen if you eat foods that contain harmful germs.
The circular muscle located at the entrance and exit to the stomach is called the sphincter. So for this question, you are talking about two distinct sphincters. This anatomical structure is able to open, and contract or close as needed.
A. A. At the entrance to your stomach is a valve, which is a ring of muscle called the lower esophageal sphincter (LES). Normally, the LES closes as soon as food passes through it. If the LES doesn't close all the way or if it opens too often, acid produced by your stomach can move up into your esophagus.