Esophagus

The esophagus is a muscular tube found in vertebrates that carries food from the throat to the stomach using wave-like contractions. In adult humans, it is about 25 centimeters long and made of fibromuscular tissue. It runs behind the windpipe and the heart, goes through the diaphragm, and connects to the top of the stomach. This organ is part of the upper digestive system, and its upper section contains taste buds. When swallowing, the epiglottis folds backward to block food from entering the airway. The word "esophagus" comes from ancient Greek, combining a future form of "to carry" with "I ate."

The wall of the esophagus has several layers: an inner mucosa, a submucosa of connective tissue, muscle fibers sandwiched between fibrous tissue, and an outer connective tissue layer. The mucosa is made of stratified squamous epithelium, about three cells thick, which differs from the single layer of columnar cells in the stomach; the border between them appears as a zig-zag line. Most of the muscle is smooth, though the upper third is mostly striated muscle. Two muscular rings, or sphincters, sit at the top and bottom of the esophagus. The lower sphincter helps stop stomach acid from flowing back up. The esophagus has a rich blood supply and venous network. Its smooth muscle is controlled by involuntary nerves—sympathetic nerves from the sympathetic trunk and parasympathetic nerves from the vagus nerve—while its striated muscle receives voluntary nerve signals from lower motor neurons carried by the vagus nerve.

The esophagus starts at the back of the mouth, around the level of the sixth cervical vertebra behind the cricoid cartilage, then passes through the rear of the mediastinum and the diaphragm, ending at the stomach’s cardia near the eleventh thoracic vertebra. Only about one centimeter of it lies in the abdominal cavity. Blood supply varies along its length: the upper part and upper sphincter get blood from the inferior thyroid artery; the thoracic portion from bronchial arteries and branches of the thoracic aorta; and the lower part and lower sphincter from the left gastric and left inferior phrenic arteries. Venous drainage also differs: the upper and middle parts drain into the azygos and hemiazygos veins, while the lower part drains into the left gastric vein. Most of these veins empty into the superior vena cava, except the left gastric vein, which goes to the portal vein. Lymph drains from the upper third into deep cervical nodes, the middle into superior and posterior mediastinal nodes, and the lower esophagus into gastric and celiac nodes, similar to other foregut structures.

The upper esophagus sits behind the trachea in the mediastinum, near the tracheoesophageal stripe, and in front of the erector spinae muscles and spine. The lower esophagus lies behind the heart and curves in front of the thoracic aorta. Below the tracheal bifurcation, it passes behind the right pulmonary artery, left main bronchus, and left atrium before going through the diaphragm. The thoracic duct, which drains most of the body’s lymph, runs behind the esophagus, shifting from the right side in the lower part to the left side in the upper part. The esophagus also lies in front of parts of the hemiazygos and intercostal veins on the right. The vagus nerve splits and forms a plexus around the esophagus.

The esophagus has four narrow points where swallowed objects or corrosive substances are likely to get stuck: at its start behind the cricoid cartilage; where the aortic arch crosses it in the superior mediastinum; where the left main bronchus compresses it in the posterior mediastinum; and at the esophageal hiatus where it passes through the diaphragm. Two sphincters, the upper and lower esophageal sphincters, close the esophagus when food is not being swallowed. The upper sphincter is an anatomical ring formed by muscle.

The esophagus can be affected by conditions like gastric reflux, cancer, enlarged veins called varices that may bleed heavily, tears, strictures, and motility disorders. These diseases may cause difficulty swallowing, painful swallowing, chest pain, or no symptoms at all. Doctors investigate using X-rays with barium sulfate, endoscopy, and CT scans. Surgically, the esophagus is hard to reach because it lies between critical organs, directly behind the sternum and in front of the spinal column.