Digestive System

After the food is swallowed, it passes the opening to the trachea and travels into the esophagus. The esophagus is a thin tube that connects the pharynx  with the stomach.  The function of the esophagus is to transport food from the throat to the stomach after it is swallowed.

When the bolus passes through the pharynx, a sphincter muscle at the entrance of the esophagus opens up and allows the bolus to enter. A sphincter muscle is a muscle that is wrapped around the entrance of the esophagus like a rubber band. Usually, the rubber band is tight. However, when the bolus reaches the esophagus, the rubber band stretches and allows the bolus to enter. This happens because the sphincter muscle, which is usually contracted, relaxes. The sphincter muscle also opens because the muscles that are attached to the hyoid bone contract, pulling the hyoid bone and the larynx forward. The pressure of the bolus against the sphincter muscle also helps to open it.

After the bolus passes the epiglottis, the esophagus stretches. When the bolus enters the esophagus, the muscles in the muscularis externa layer of the esophagus initially relax around the food. The sub-layer of circular muscle above the bolus contracts, causing the cavity within the tract to narrow. Then the longitudinal sub-layer below the bolus contracts, which causes a section of the esophagus to shorten and widens the cavity within the esophagus. This action, which is called peristalsis action, occurs in waves and forces the food toward the stomach. Although gravity mainly helps the bolus to get to the stomach if the person who ate it is sitting or standing upright, peristalsis action is so strong that it causes the bolus to travel to the stomach even if the person who ate the food is standing on his or her head.