What are the muscles that guard entrances and exits of internal passageways?

The muscles that guard entrances and exits of internal passageways are called sphincter muscles.

Examples include:

Oral sphincter: Found at the opening of the mouth, it controls the entry of food into the oral cavity.

Anal sphincter: Located at the end of the rectum, it helps to control the release of feces.

Urethral sphincter: Situated around the urethra, it aids in regulating urine flow.

Pyloric sphincter: Present at the outlet of the stomach, it controls the passage of food into the small intestine.

Esophageal sphincter: Also called the lower esophageal sphincter (LES), it is found at the junction of the esophagus and stomach, preventing the backflow of stomach contents.