The esophagus is a muscular tube that transports food and liquid from the pharynx down to the stomach. This cylindrical passageway acts as a controlled conduit, ensuring that the swallowed contents, known as a bolus, are efficiently delivered past the windpipe and into the digestive tract.
Physical Appearance and Location
The esophagus is a flexible, fibromuscular tube that averages about 25 centimeters (10 inches) in length and measures approximately 1 to 2 centimeters in diameter when distended. It is situated in the center of the chest, traveling behind the trachea (windpipe) and the heart, and in front of the spinal column. It must pass through an opening in the diaphragm before it connects to the stomach.
The esophageal wall is composed of four distinct tissue layers. The inner lining, or mucosa, is made of stratified squamous epithelium, a durable tissue that provides protection as food passes. Surrounding this is the submucosa, containing blood vessels and glands that produce mucus for lubrication. The muscular layer, known as the muscularis propria, consists of an inner circular layer and an outer longitudinal layer.
The muscle composition changes along the length of the tube. It starts with voluntary striated muscle in the upper third, transitions to a mixture of striated and smooth muscle in the middle third, and ends with smooth muscle in the lower third.
The outermost layer, called the adventitia, is a loose connective tissue that anchors the structure to surrounding organs. When the esophagus is empty, the mucosal and submucosal layers fold inward, causing the lumen, or channel, to appear star-shaped in cross-section.
How the Esophagus Moves Food
The primary action of the esophagus is peristalsis, a rhythmic wave of muscle contraction that propels the food bolus toward the stomach. Swallowing initiates this process as a voluntary action, but once the bolus enters the esophagus, the movement becomes an involuntary reflex. This movement, known as primary peristalsis, is a continuation of the contraction wave that begins in the pharynx.
During a peristaltic wave, the circular muscle layer contracts behind the bolus, squeezing it downward, while the longitudinal muscle layer simultaneously contracts to shorten the segment of the tube. Just ahead of the bolus, the circular muscles relax, allowing the passageway to open. This sequential contraction and relaxation ensures the food is pushed reliably to the stomach.
If the initial wave of primary peristalsis fails to clear all the contents, a second wave, called secondary peristalsis, is triggered. This secondary wave is stimulated not by swallowing, but by the local distension caused by the remaining food or liquid. This reflex clears the residual material, ensuring that the esophagus is empty and ready for the next swallow.
Specialized Valve Systems
The function of the esophagus relies on two muscular rings, or sphincters, that act as one-way valves. The Upper Esophageal Sphincter (UES) is located at the top and is composed of striated muscle. The UES remains tightly closed when a person is not swallowing, preventing the intake of air during breathing and blocking the backflow of material into the pharynx.
This top valve prevents aspiration, which is the entry of food or liquid into the airway. During swallowing, the UES relaxes rapidly, allowing the bolus to enter the esophageal body, and then immediately closes once the food has passed. The Lower Esophageal Sphincter (LES) is a high-pressure zone at the junction where the esophagus meets the stomach.
The LES is composed of smooth muscle and is considered a functional sphincter because it lacks the distinct thickening of other muscular rings. Its main role is to relax when the peristaltic wave arrives, allowing the food to pass into the stomach. The LES remains shut to form a barrier that blocks the acidic contents of the stomach from refluxing into the esophageal lining.