食道

The esophagus is a muscular conduit approximately 25 cm long that transports food from the mouth to the stomach through peristaltic contractions. It facilitates the movement of the food bolus while esophageal mucosa secretes lubricating substances to ensure smooth passage. The esophagus also prevents stomach contents from refluxing back into the throat through the lower esophageal sphincter.

The lower esophageal sphincter, also called the gastroesophageal or cardiac sphincter, is a thickened ring of circular smooth muscle surrounding the cardial orifice. It remains contracted to seal the stomach and opens only during swallowing to allow food passage. The diaphragm muscle encircles this sphincter, assisting in maintaining the seal when swallowing is not occurring, protecting the esophagus from gastric acid backflow.

The esophagus comprises four foundational layers: the mucosa with nonkeratinized stratified squamous epithelium, the submucosa containing esophageal glands that produce mucus, the muscularis externa made of skeletal and smooth muscle, and the fibrous adventitia composed of connective tissue. Unlike other digestive organs, the esophagus lacks a serosa layer and instead has an adventitia.

The esophagus transitions from skeletal muscle in its proximal third to a mixture of skeletal and smooth muscle in the middle third, and finally to smooth muscle only in the distal third. This gradual transition allows the esophagus to initiate voluntary swallowing through skeletal muscle while completing involuntary peristalsis through smooth muscle contractions that propel food toward the stomach.

The esophageal mucosa consists of nonkeratinized stratified squamous epithelium with basal and parabasal cells that provides protection against wear and tear from food particles. The lamina propria beneath this epithelium houses glands that secrete mucus, which lubricates the esophagus and facilitates smooth food passage without damaging the delicate tissue.

Incomplete closure of the lower esophageal sphincter allows reflux of stomach contents into the esophagus, causing conditions like heartburn or gastroesophageal reflux disease (GERD). This occurs when the sphincter does not maintain adequate pressure to prevent chyme from entering the esophagus, exposing the esophageal lining to gastric acid.

The esophagus is unique among digestive organs because its outermost layer is the adventitia, composed of loose connective tissue, rather than the serosa found on the stomach and intestines. Additionally, the esophagus remains collapsed when not swallowing, courses through the mediastinum, and penetrates the diaphragm through the esophageal hiatus before connecting to the stomach.