Although your esophagus is essentially a hollow tube, its walls are composed of a number of highly-specialized layers, including an inner lining made up of thin, flat cells (squamous cells), a layer below the inner lining (submucosa) that contains mucus-secreting glands, and a thick band of muscle tissue.

When you eat or drink, a muscle in the upper part of your esophagus (upper esophageal sphincter) relaxes, allowing food and liquid to enter. Smooth muscles in the esophagus wall then move the food along in a series of rhythmic contractions — a process called peristalsis. It usually takes four to 10 seconds for food to flow through your esophagus.

Another ring of muscle, the lower esophageal sphincter, sits at the junction where your esophagus and stomach connect. It opens to allow food into your stomach and then clamps shut so that corrosive stomach acids and digestive enzymes don't back up into the esophagus.

Cancer can occur almost anywhere along the length of the esophagus and is classified according to the types of cells in which it originates:

• Squamous cell or epidermoid carcinoma. The most common esophageal cancer in black Americans, squamous cell carcinoma develops in the flat squamous cells that line the esophagus.

• Adenocarcinoma. This arises in the glandular tissue in the lower part of the esophagus nearest the stomach.

• Others. Although squamous cell and adenocarcinoma are the primary types of esophageal cancer, other, rare forms of the disease sometimes occur. These include sarcoma, lymphoma, small cell carcinoma and spindle cell carcinoma. In addition, cancer that starts in the breast or lung can spread (metastasize) through the bloodstream or lymph system to the esophagus.

Healthy cells grow and divide in an orderly way. This process is controlled by DNA — the genetic material that contains the instructions for every chemical process in your body. When DNA is damaged, changes occur in these instructions. One result is that cells may begin to grow out of control and eventually form a tumor — a mass of malignant cells.

Although researchers don't know all the causes of esophageal cancer, they have identified several factors that can damage DNA in your esophagus. These factors include:

• Heavy alcohol consumption. Esophageal squamous cell carcinomas is a result from chronic alcohol abuse. Long-term heavy drinking irritates the lining of the esophagus, leading to inflammation that eventually may cause malignant changes in the cells.

• Tobacco use. Using tobacco in any form, including cigarettes, cigars, pipes and chewing tobacco, increases the likelihood of developing esophageal squamous cell carcinoma. The risk increases with long-term use and rises dramatically for people who both smoke and drink.

• Chronic acid reflux. Sometimes the lower esophageal sphincter relaxes abnormally or weakens, allowing caustic stomach acids to back up into your esophagus (esophageal reflux). The result is heartburn — a burning chest discomfort that in severe cases may mimic the symptoms of a heart attack. Occasional heartburn usually isn't serious, but chronic acid reflux can lead to Barrett's esophagus, a condition in which cells similar to the stomach's glandular cells develop in the lower esophagus. These new cells are resistant to stomach acid, but they also have a high potential for malignancy.

• Chemical irritation. Each year, nearly 1 million children under age 5 accidentally drink toxic household substances. Some of these chemicals, especially drain cleaners that contain lye, burn the lining of the esophagus and may contribute to esophageal cancer later in life.

• Diet. Eating a diet low in fruits and vegetables appears to contribute to esophageal cancer. Especially implicated are diets lacking in vitamins A, C, B1 (riboflavin), the mineral selenium, and beta-carotene — a substance found especially in orange and yellow fruits and vegetables that is converted into vitamin A in your body.

• Obesity. Weighing 20 to 30 pounds more than your ideal weight has been linked to an increased risk of adenocarcinoma.

• Barrett's esophagus

• Obesity

• sarcoma

Sometimes esophageal cancer is associated with certain rare medical conditions, including:

• Achalasia. In this disorder, food collects at the bottom of the esophagus, both because the esophagus lacks normal peristalsis to move food along and because the lower esophageal sphincter doesn't relax normally. For reasons that aren't clear, having achalasia seems to increase your risk of esophageal cancer.

• Esophageal webs. These thin protrusions of tissue can appear anywhere in the esophagus. Some webs cause no symptoms, but others can make swallowing difficult. When other problems — including anemia and abnormalities of the tongue, fingernails and spleen — occur in conjunction with esophageal webs, the condition is called Plummer-Vinson or Paterson-Kelly syndrome. People with this syndrome are at risk of developing esophageal cancer.

• Tylosis. This rare, inherited disorder causes excess skin to form on the soles and palms. Close to half the people with tylosis eventually develop esophageal cancer. A genetic defect appears to be responsible for both tylosis and the associated cancer.

Heavy drinking and smoking are the two greatest risk factors for esophageal squamous cell carcinoma. The risk increases substantially if you drink as well as smoke. If you drink heavily every day for several years, your risk of esophageal cancer is 18 times greater than it is for someone who drinks in moderation or not at all. When you also smoke, your risk nearly doubles.

• Barrett's esophagus

• Chest pain

• Nicotine dependence

• overweight

Other risk factors for esophageal cancer include:

• Age. Your risk of developing esophageal cancer increases as you grow older. Most people with the disease are between 45 and 70. The risk is much less if you're under 40.

• Sex. Men are three times as likely to develop esophageal cancer as women are.

• Race. Squamous esophageal cancer affects three times as many black Americans as whites, whereas whites have much higher rates of esophageal adenocarcinoma than do blacks. Although the reason for this disparity isn't known, genetic factors may play a role.

• Diet. If your diet is low in fruits and vegetables, or you're very overweight, you're at increased risk of esophageal cancer.

• Chronic heartburn or Barrett's esophagus. Both conditions, which occur when stomach acid backs up into your esophagus, increase your risk of esophageal cancer.