What are the two distinct muscles that close off the esophagus called?

The two distinct muscles that close off the esophagus are referred to as the upper and lower esophageal sphincters. The upper esophageal sphincter is located at the junction of the pharynx and esophagus, functioning to prevent the backflow of food and liquids, while the lower esophageal sphincter is situated at the junction of the esophagus and the stomach, playing a critical role in preventing gastric contents from refluxing back into the esophagus. This mechanism is essential for maintaining a one-way flow of food from the throat to the stomach and protecting the esophagus from acidic contents that could cause irritation or damage.

The other options pertain to different sphincters located in other parts of the body, such as the cardiac and pyloric sphincters, which are involved in the regulation of stomach contents, and the internal and external anal sphincters that control bowel movements. The thoracic and abdominal sphincters are not standard terms used in anatomy and do not specifically describe any known muscles related to the esophagus. Thus, the identification of the upper and lower esophageal sphincters accurately specifies the muscles that function to close off the esophagus.