Esophagus

The esophagus, a muscular conduit linking the pharynx and stomach, measures roughly 10 inches (25.4 cm) and sits behind the trachea. It remains collapsed when not swallowing. The esophagus follows a predominantly straight path through the thoracic mediastinum and enters the abdominal cavity through a diaphragmatic opening known as the esophageal hiatus.

The movement of edibles from the pharynx into the esophagus is facilitated by the upper esophageal sphincter, which is formed primarily by the cricopharyngeus muscle. The esophagus's proximal third is composed mainly of skeletal muscle, and the middle third is a mixture of smooth and skeletal muscle tissue; however, the skeletal muscles gradually disappear in the esophagus's final third. In the esophagus, peristaltic rhythmic contractions help propel the food bolus toward the stomach. Concurrently, the esophageal mucosa excretes substances that lubricate the esophagus and the food. The lower esophageal sphincter (also termed the gastroesophageal or cardiac sphincter) is the passage from the esophagus into the stomach. Sphincters function as valves, opening, and closing to regulate the flow through tubes. The lower esophageal sphincter relaxes, lets food go into the stomach, and then contracts to prevent the stomach's contents from refluxing into the esophagus. The diaphragm muscle encircles this sphincter, assisting in sealing off the sphincter when swallowing is not occurring. Incomplete closure of the lower esophageal sphincter can result in reflux of stomach contents into the esophagus, causing conditions like heartburn or gastroesophageal reflux disease (GERD).

The esophagus's mucosal layer consists of an epithelial lining with stratified squamous epithelium, which is not keratinized, and a stratum of basal and parabasal cells. This type of epithelium offers protection against food particle-induced wear and tear. The lamina propria of the mucosa houses glands that secrete mucus. The composition of the muscularis layer varies with its anatomical location: the upper third comprises skeletal muscle, the middle third is a mix of skeletal and smooth muscles, and the lower third consists only of smooth muscles. Contrary to the stomach and intestines, the outermost layer of the esophagus, known as the adventitia instead of the serosa, is not enveloped by a visceral peritoneum fold. This layer is mostly comprised of loose connective tissue.

The digestive functions of the esophagus include allowing the food bolus to pass from the laryngopharynx to the esophagus, peristalsis for propelling the bolus through the esophagus, relaxation of the lower esophageal sphincter to enable the bolus to move from the esophagus to the stomach and to prevent chyme from entering the esophagus, and mucus secretion for lubricating the esophagus to aid smooth passage of the bolus.