The digestive system, also known as the gastrointestinal or GI system, is responsible for digestion, one of the most important functions of the body. Digestion breaks down food into nutrients that can be absorbed by the body for fuel, growth and cell repair.

Digestion takes place in the alimentary canal, also called the digestive or gastrointestinal tract, over a period of several hours. The alimentary canal consists of a series of organs that runs from the mouth to the anus and includes the esophagus, stomach, intestines, colon and rectum.

The digestive system canal performs four essential functions:

• Propels food from the mouth through the esophagus and eventually out the rectum. During this process, the food is continuously mixed and broken down into a fluid.
  
  
• Releases secretions of fluids and chemicals that help in digestion and absorption. These secretions are produced in the salivary glands (located underneath the tongue), pancreas and liver as well as the linings of organs such as the stomach and small intestines.
  
  
• Breaks down ingested foods into absorbable molecules. Although there are many varieties of foods that people eat, the ingredients in them are broken down into similar components by the digestive system. Proteins are broken down into amino acids. Carbohydrates (starches and sugars) are broken down into simple sugars (e.g., glucose). Fats are broken down into fatty acids and glycerol (a type of sugar alcohol).
  
  
• Absorbs nutrients, water and electrolytes from the intestinal lumen (inner open cavity) into the bloodstream.

The functions of the digestive system are controlled by hormones that are produced and released by cells in the lining of the stomach and small intestine. The hormones that control digestion include:

• Gastrin. This hormone directs the stomach to produce an acid for dissolving and digesting foods (stomach acid). In addition, gastrin also helps the normal growth of the stomach lining, small intestine and colon.
  
  
• Secretin. A hormone that prompts the pancreas to send out a digestive juice that is rich in sodium bicarbonate, a substance that neutralizes stomach acid. It also stimulates the stomach to produce pepsin, an enzyme that helps digest proteins as well as stimulate the liver to produce bile.
  
  
• Cholecystokinin (CCK). This hormone causes the pancreas to grow and produce the enzymes of pancreatic juice. CCK also causes the gallbladder to empty.

Hormones in the digestive system also regulate appetite. In the absence of food, ghrelin is produced in the stomach and stimulates hunger. After eating, the hormone peptide YY is produced in the intestines and acts as an appetite suppressant. The combination of these hormones helps the body regulate food intake.