Food is digested and absorbed in the alimentary canal, a tube of organs from the mouth to the anus that is about 30 feet (9 meters) long in the average adult. The main features of the alimentary canal and the role of its organs during digestion include the following, from top to bottom:

• Mouth. The opening in the head through which food enters the body. Even before a person takes an initial bite of food, the sensations provided by sight, smell and even imagination of appetizing food prompts the salivary glands under the tongue to secrete saliva. This prepares the body for the meal to come.
  
  As the teeth grind food into smaller bits, saliva moistens it for easier swallowing. A digestive enzyme in the saliva known as amylase breaks down carbohydrates (sugars and starches). Muscle movements in the tongue and mouth provide the swallowing necessary to push the food into the throat (pharynx). This is a passageway for food and air that is about 5 inches (12.7 centimeters) long. A flexible flap of tissue called the epiglottis reflexively closes over the windpipe during swallowing to prevent choking and to prevent food from entering the lungs.
  
  
• Esophagus. As the food travels down the throat, it arrives at a junction just above a long, muscular tube known as the esophagus. This junction, called the upper esophageal sphincter, is a muscular valve that opens to allow food into the esophagus. A series of rhythmic contractions in the esophageal wall moves the food down the tube. This process is called peristalsis, and it takes between four and 10 seconds in the esophagus. The muscular movement of peristalsis continues throughout the digestive system. At the end of the esophagus, a muscular ring called the lower esophageal sphincter opens and allows food to enter the stomach. Then it quickly closes to keep the stomach’s digestive juices from flowing back into the esophagus and damaging the tube’s walls.
  
  
• Stomach. A large, bean-shaped, hollow muscular organ consisting of three regions: the cardia, the body (fundus) and the antrum. The upper stomach serves as a storage area for food. Here, the cardia and fundus of the stomach relax to make room for food that enters the stomach. Once food enters the stomach, muscles begin to churn and mix the food with acids and enzymes (gastric juices). This breaks down the food into smaller, digestible pieces. Cells in the stomach lining secrete mucus, hydrochloric acid and pepsin, which aid in this process.
  
  Mucus protects the cells of the stomach lining from damage by the acid and enzymes. Any disruption to this layer of mucus, such as from a bacterial infection for example, can result in damage that leads to a stomach ulcer. Hydrochloric acid, also called stomach acid, is produced by parietal cells in the stomach lining and provides the highly acidic environment necessary for pepsin to break down proteins.
  
  At this point, some substances (e.g., water, salt, sugars and alcohol) are ready to be absorbed into the body directly through the stomach wall. However, most substances still require further digestion. Once the food has been processed into a thick liquid called chyme, it is stored in a walnut-sized muscular tube at the bottom of the stomach called the pylorus. Once the chyme achieves the right consistency, a muscle between the stomach and small intestine called the pyloric sphincter opens to allow its passage into the small intestine.
  
  
• Small intestine. Chyme secreted from the stomach moves down into the small intestine, which is made up of three parts:
  
  
• • Duodenum. The C-shaped first part.
  • Jejunum. The coiled midsection.
  • Ileum. The final section leading into the large intestine.

Digestion of chyme continues as it is further broken down in the small intestine until the nutrients can be absorbed into the bloodstream. Millions of microscopic, finger-like projections (villi) cover the wall of the small intestine. Nutrients are absorbed through these villi.

• Large intestine. Undigested food and some water moves into the large intestine through another muscular ring that prevents food from returning to the small intestine. The large intestine removes water from the undigested matter and solid waste that can be excreted. The large intestine is made up of three parts:
  
  
• • Cecum. Pouch that joins the small intestine to the large intestine. The appendix is an even smaller pouch attached to the cecum. The appendix has no known function, but can become inflamed and require surgical removal.
    
    
  • Colon. A long tube that connects the cecum to the rectum. Bacteria here help digest any remaining food. The ascending colon and transverse colon absorb fluids and salts. The descending colon receives the resulting waste. The sigmoid colon empties the waste into the rectum.
    
    
  • Rectum. Area below the colon where feces are stored until they are ready to be expelled from the body through the anus as a bowel movement.

  

It takes between six and eight hours after a person consumes food before it passes through the small intestine and stomach. Elimination of food residue from the colon usually begins after 24 hours, with complete elimination taking up to several days.

Accessory digestive organs are not part of the alimentary canal, but play an essential role in the digestive process. These include:

• Pancreas. Located beneath the stomach, it produces enzymes that help digest proteins, fats and carbohydrates. The pancreas also makes sodium bicarbonate, which neutralizes stomach acid.
  

  

• Liver. Located just beneath the rib cage in the right upper part of the abdomen, it produces bile, which helps the body absorb fat. The liver also regulates substances in the blood cells.
  

  

• Gallbladder. Located just below the liver, it stores bile until it is needed.