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.
The stomach is divided into 5 regions: The cardia is the small area in the upper portion of the stomach near the esophagus. This region includes the cardiac sphincter, which is a ring of muscle where the esophagus and stomach join. The cardiac sphincter acts as a valve that prevents the stomach contents from going back up into the esophagus.