For food and liquids to travel from the mouth to the stomach, they must pass through the esophagus. Made of both fibrous and muscular tissue, the esophagus is about ten inches long and stretches from the throat to the top of the stomach. Mucus that lines the esophagus provides lubrication for the swallowed material. Additionally, contractions push residual material into the stomach. Most people are never aware of their esophagus until they swallow something that is too hot or cold.
The esophagus begins in the back of the mouth. Like the tongue, the upper portion of the esophagus has many taste buds. From the mouth, it passes through the compartment of the torso that houses the heart and other thoracic vessels. Then it passes the diaphragm, a sheet of muscle that separates the thoracic cavity from the abdominal cavity. In the abdominal cavity, the esophagus connects to the stomach. A branching network of nerves begin at the vagus nerve and cover the esophagus, controlling the heart, lungs, and digestive tract.
Two muscular rings surround the top and bottom of the esophagus. These are the upper and lower esophageal sphincters. They are responsible for closing the esophagus when an individual isn’t swallowing anything. The swallowing reflex controls whether the sphincters are open or closed. The lower esophageal sphincter works alongside the crural diaphragm to ensure no gastric acid from the stomach damages it. Striated muscles compose the upper sphincter, while smooth muscles make up the lower sphincter.
Three muscles and several structures -- the rear portion of the thyroid and the hyoid, a horseshoe-shaped bone in the neck -- form the upper esophageal sphincter. The three muscles are the cricopharyngeus, thyropharyngeus, and cranial cervical esophagus. These three muscles each have different responsibilities that allow the upper esophageal sphincter to operate. They receive input signals as part of the swallowing reflex that allows the sphincter to open when needed.
Two components combine to form the lower esophageal sphincters: an intrinsic component and an extrinsic component. The intrinsic portion consists of muscle fibers that respond to molecules called neurohormones that carry signals from the brain. The extrinsic component is a diaphragm muscle that controls the pressure in the lower sphincter. Anything that malfunctions or interrupts the closing of the lower esophageal sphincter can lead to gastroesophageal reflux and changes in the mucus lining of the esophagus.
A mucous membrane in the esophagus lubricates the tube for the passage of food. This membrane is made of a stratified squamous epithelium, which means it possesses flattened cells that allow for secretion and absorption. The membrane also features a lamina propria, a thin layer of connective tissue that provides support for the epithelium and binds it to the underlying tissue. Finally, the muscularis mucosae lie outside the lamina propria. This layer of muscle is in a constant state of gentle movement that helps the membrane secrete mucus.
After a person chews their food, the mouth passes it through the pharynx into the esophagus. Here, peristaltic contractions of striated muscle propel the food downward. The upper esophageal sphincter then relaxes to allow the food through. In the center of it, a combination of striated and smooth muscles work together to push the food to the lower esophageal sphincter. The sphincter relaxes, allowing the food to enter the stomach. The muscle contractions occur as a reflex when the body senses food in the mouth, and to any sensation of food in the esophagus itself.
The stomach produces a strong acidic mixture that allows it to digest food. This is the gastric acid. The lower esophageal sphincter helps ensure the acid remains in the stomach, keeping it from entering the esophagus and scarring it. Scarification can lead to constriction of the tube and trouble swallowing. If this happens frequently, doctors may diagnose gastroesophageal reflux disease. People with this disease experience recurring heartburn and chest pain.
In extreme cases of gastroesophageal reflux disease, tissue similar to the intestinal lining replaces the tissue normally present in the mouth and esophagus. This is a condition experts call Barrett’s esophagus. People with this condition have a higher chance of developing esophageal cancer. Beyond this, they experience only symptoms similar to gastroesophageal reflux disease. There are cases where people without a reflux disease or disorder develop Barrett’s esophagus, and the cause is unknown.
Any portion of the esophagus can become cancerous. Typically, the cells lining the interior are the first to be affected. Common symptoms of esophageal cancer are difficulty swallowing, chest pain, heartburn, and coughing. Several types of cancer can affect it. The two most common are adenocarcinoma, which begins in the mucus-secreting glands, and squamous cell carcinoma, which begins in the flattened cells of the esophagus.
The veins in the esophagus can become enlarged when a clot or scar tissue blocks the blood flow to the liver. The blood then flows into smaller vessels that aren't capable of transferring large volumes of blood without issue. These become esophageal varices and risk rupturing, causing life-threatening internal bleeding. Esophageal varices occur most often in people with liver disease. Symptoms of esophageal varices include vomiting, bloody stools, and lightheadedness. The symptoms of liver disease are jaundice and easy bruising.
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