Introduction to The Esophagus
As humans, we derive our energy from the foods that we eat. The long journey that food takes so that our bodies can absorb the energy is called digestion. Digestion is a fascinating process that involves over ten different organs. In this article we will discuss the esophagus. If the mouth and the oral cavity is the gate that opens to begin this journey the esophagus is the road that leads to energy absorption. It is essentially a super highway. While it typically takes around 24 hours to completely digest a meal, it only takes about 7 seconds for solid food to pass through the esophagus and into your stomach and only around 2-3 seconds for liquids.
Location of the Esophagus
The esophagus is a hollow tube that begins at the back of the mouth at around the sixth cervical vertebrae. The esophagus runs behind the trachea. When we swallow food or liquids the epiglottis falls back and covers the larynx, sort of like a railroad switch, so food does not travel down into the bronchial passages of the lungs. This upper portion of the esophagus has taste buds which is what causes the “after taste” effect. From here the esophagus travels down behind the heart and in front of the spine and eventually passes through the diaphragm to finally end at the stomach.
Picture 1: Location of the esophagus in the human body
In an adult, the esophagus is usually around 25 to 30 centimeters in length and can measure up to about 2 centimeters in width. The esophagus can also widen on its own to allow solids to pass through more easily. As described above there are 3 areas of narrowing that occur when the esophagus encounters a structure that belongs to a different system of the body. In a healthy, normally developed individual these points of narrowing keep from disrupting the functions of other systems such as the trachea, the aorta, and the respiratory system.
In early development of an embryo, the esophagus begins forming in the endodermal portion of the embryonic cells. There are three layers of embryonic cells:
- The Ectodermal Layer: This is the outermost layer of an embryo. In this layer of an embryo skin and pigment cells begin to develop.
- The Mesodermal Layer: This middle layer develops different types of muscle tissues, and later on is where the development of the reproductive organs occurs.
- The Endodermal Layer: This is the deepest layer of the cells and is where development of most internal organs, including the esophagus, takes place.
The whole Gastrointestinal tract starts off as a single tube, called the primitive gut. This usually begins developing around the fourth week of the life of an embryo. During the development of the primal gut we begin to see a structure that will be be the esophagus.
Esophagus Layers and Anatomy
The esophagus is made up of four layers of tissue. These layers are similar all throughout the whole digestive tract. The order of these layers from the inside out are:
Picture 2: Different layers of the esophagus
There are three layers of the mucosa:
- The Epithelium: The inside of the esophagus is made up of cells called stratified squamous epithelium.
- The Lamina Propria: This is a layer of connective tissue just under the epithelium. This layer contains lymphocytes. Cells that are an important part of our immune system.
- Muscularis Mucosa: This is a layer of smooth muscle that is essential in helping nutrients flow into the submucosa.
This is layer where nutrients are passed from the mucosa and introduced into the bloodstream. The mucosa and submucosa shouldn’t really be seen as two separate layers, but instead two parts that make a whole. While they work together the submucosa houses nerves that are responsible for peristalsis (or muscle contractions, more on this later..) and also lymphatic fluids that aid in the immune system.
This is the layer of muscle on the outside of the esophagus. Nerves from the submucosal layer travel to these muscles to create peristalsis, “wavelike” contractions, that move food and fluids in the proper direction towards the stomach.
The adventitia is a membrane on the outside of any organ that is not covered by a serous membrane, such as the stomach or the chest cavity. Adventitia is loose connective tissue while serosa is a thin connective tissue that emits serous fluids.
The esophagus is divided into three sections. The cervical section, thoracic section and the abdominal section. In the cervical section the Upper Esophageal Sphincter (UES) acts as a defense against secretions that may try to run down the trachea. It opens when we burp or vomit and also when we eat. Sort of like the epiglottis acting to keep anything but air entering the trachea, the Upper Esophageal Sphincter acts to keep air from entering the stomach when we breath.
In the cervical section of the esophagus we also encounter the first of three areas where the esophagus narrows. This occurs near the cricoid cartilage. The next area of narrowing occurs in the thoracic section where it is passed over by the left main bronchi of the lungs and the aorta, the largest artery of the body. The last area of narrowing occurs at the end of the thoracic section and near the beginning of the abdominal section where it passes through the diaphragm at what is called esophageal hiatus.
The esophagus then ends at the Lower Esophageal Sphincter (LES). The Lower Esophageal Sphincter is a group of involuntary muscles that keep stomach acid from entering the esophagus by staying closed unless we swallow.
Functions of the Esophagus
As we have seen, the esophagus serves as the direct route for foods and liquids to enter the stomach. But it is not simply gravity that makes these materials slide down into the stomach. Peristalsis, the “wavelike” contractions that are made by the thick layer of the muscularis propria, push food in the appropriate directions. These contractions are so great that it is possible, though not advisable, to eat upside down and for astronauts to eat and drink in zero gravity environments. Peristalsis also occurs in the intestinal tract. Peristalsis is created by circular muscles in the esophagus. If you have ever seen a worm moving forward you have witnessed what is pretty close the the contractions of the muscles in a gastrointestinal tract.
While the idea is to move things (food) forward, sometimes something that is called retroperistalsis happens. As the name suggests this happens when peristalsis moves in reverse. The is a defense mechanism that usually results in vomiting.
Diseases and Disorders Associated
As we can see, the esophagus is exposed to a lot. It carries food, drinks, and is often one of the first lines of defense when it comes to our immunity. Because of this it can be the subject of many types of disorders:
- Motility Disorders: These disorders involve the movement of objects through the esophagus. Typically a person with motility disorders will have trouble swallowing food or have trouble with the esophagus moving the food to the stomach. There are many things that may cause motility disorders including muscular disorders or excessive use of drugs and alcohol.
- Acid Reflux: A very common problem among adults and even children, acid reflux occurs when stomach acid spews up into the esophagus. The chronic condition of the is called Gastroesophageal Reflux Disease or GERD. Because the esophagus passes so close to the heart pain from acid reflux can often mimic a heart attack.
- Eosinophilic Esophagitis: Often a by product of repeated damage done to the esophagus through acid reflux or by other means, EOE is when certain types of white blood cells (eosinophils) attack the tissues of the esophagus. This generally leads to problems such as motility disorders and sometimes wearing away of esophageal tissues..
- Spasms: Like other muscles of the body the muscles of the esophagus are prone to spasms. Muscle spasms can be caused by a number of things including electrolyte imbalances, dehydration, or muscle disorders.
- Esophageal Varices: When the veins of the submucosal layer become engorged they are called varices. This is a very serious condition that may present no signs or symptoms until a vein bursts. When this happens, a person will typically vomit or spit up blood and may become light headed because of high amounts of internal bleeding. This is usually a sign of other big problems such as liver cirrhosis.
- Esophageal Cancer: The “Big C”. Esophageal cancer ranks number six of the cancers that cause death. The inner lining of the esophagus is made from striated squamous cells. When these cells mutate and cause masses we see a cancer that is called squamous cell carcinoma. This is the most common type of esophageal cancer in the world.
As we have seen the esophagus is a very interesting organ. It plays a vital role in digestion while contorting itself to make room for other body systems. The esophagus is often an overlooked part of the GI system where the stomach and the intestines seem to outshine their superiorly placed friend because of their roles in energy absorption and digestion . But without the esophagus, the stomach and intestines would have nothing to work with. Without the esophagus to deliver food into our stomachs the only other way to receive nutrients would be through medical intervention with the use of nasogastric tubes or even more radical equipment like G-tubes that go directly into the stomach. The esophagus is a fascinating organ that plays an integral part as the road to digestion.