Along with the mouth and throat, the esophagus is the start of a long path of food in the body. Located in front of the spine but behind the windpipe and heart, this tube's primary job is to convey food, via tiny contractions called peristalsis, from a sphincter at the bottom of the throat through another sphincter at the stomach below. Another role of the esophagus in the digestive system is to lubricate food with mucus as it makes its way down to the gut.
Before getting to the esophagus in the digestive system, food is typically well-prepared for entry. Not only is it crushed and rounded at the sharp edges by as many as 32 teeth, but it is also kneaded through by the mouth's saliva. This liquid contains the enzymes, lubricants and acid reducers needed to start reducing the food to a long malleable loaf by the time it reaches the upper esophageal sphincter at the bottom of the throat, or pharynx.
When engaged during a swallow, this initial sphincter opens while the adjacent epiglottis shuts the trachea leading to the lungs. This is the first job of the esophagus in the digestive system — keeping people from breathing food or water. Once the food has made its way into the esophagus, the sphincter closes and the epiglottis releases its hold on the trachea.
As food moves down the approximately 8-inch (approximately 0.2-meter) esophagus, it is padded by four layers of contracting tissue lined with mucus. The innermost layer is called the tunica mucosa, followed by the tunica submucosa, tunica muscularis and, finally, the tunica serosa on the outside. These outer two layers of the esophagus contract until the food reaches the lower esophageal sphincter and can enter the stomach, just below the heart. The role of the inner two layers is to moisten the food and protect the outer two layers.
Food leaves the esophagus in the digestive system's journey to enter an acidic, denaturing bath of gastric juices in the stomach, followed by the large and small intestines, then the colon and rectum. Along the way, the tract's enzymes and bacterial colonies break the food down to cellular constituents, which can then be absorbed to provide the nutrients needed for survival. The esophagus, by contrast, has no absorbing qualities.
Various disorders of the esophagus can develop, particularly for smokers or heavy drinkers. Heartburn and indigestion are common problems. Some experience damage to the esophageal sphinters, resulting in acid reflux disease. Others may develop cancer of the esophagus.