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Digestive System

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, series of connected organs whose purpose is to break down, or digest, the food we eat. Food is made up of large, complex molecules, which the digestive system breaks down into smaller, simple molecules that can be absorbed into the bloodstream. The simple molecules travel through the bloodstream to all of the body's cells, which use them for growth, repair, and energy.

All animals have a digestive system, a feature that distinguishes them from plants. Plants produce their own food in a process called photosynthesis, during which they use sunlight to convert water and carbon dioxide into simple sugars. But animals, including humans, must take in food in the form of organic matter, such as plants or other animals.

Digestion generally involves two phases: a mechanical phase and a chemical phase. In the mechanical phase, teeth or other structures physically break down large pieces of food into smaller pieces. In the chemical phase, digestive chemicals called enzymes break apart individual molecules of food to yield molecules that can be absorbed and distributed throughout the body. These enzymes are secreted (produced and released) by glands in the body.

The digestive system of most animals consists mainly of a long, continuous tube called the alimentary canal, or digestive tract. This canal has a mouth at one end, through which food is taken in, and an anus at the other end, through which digestive wastes are excreted. Muscles in the walls of the alimentary canal move the food along. Most digestive organs are part of the alimentary canal. However, two accessory digestive organs, the liver and pancreas, are located outside the alimentary canal. These organs contribute to chemical digestion by releasing digestive juices into the canal through tubes called ducts.



Digestive System -- Media -- Encarta  Online

Located on the left side of the body, under the diaphragm, the stomach is a muscular, saclike organ that connects the esophagus and small intestine. Its main function is the breakdown of food. Cells in the lining secrete enzymes, hydrochloric acid, and other chemicals to continue the digestive process begun in the mouth and produce mucus to keep these substances from digesting the lining itself

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Anatomy of the Mouth

Digestive System -- Media -- Encarta  Online

In humans, the mouth is an integral part of digestion, speech, and respiration. Food enters the mouth to be broken down both by the teeth (mechanical digestion) and by enzymes secreted by the three salivary glands illustrated here (chemical digestion). With the tongue and nasal cavity, the mouth modifies sound waves arising from the larynx to produce the sounds of speech. Air is inhaled and exhaled through the oral and nasal cavities.

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Small Intestine

Digestive System -- Media -- Encarta  Online

The small intestine is where most digestion takes place. The inner lining, or mucosa, is folded and covered with tiny fingerlike projections called villi, a design that maximizes the absorptive surface area of the intestine. Rhythmic contraction of the muscular walls moves food along while bile, enzymes, and other secretions break it down. Nutrients absorbed into the intestine's many blood vessels are carried to the liver to be distributed to the rest of the body.

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Large Intestine

Digestive System -- Media -- Encarta  Online

Anchored in the abdomen by membranes called mesenteries, the large intestine is the final section of the digestive tract. Undigested material passes from the small intestine as liquid and fiber. In the large intestine, muscular segments called haustrae shuttle this material back and forth, mixing it thoroughly. Cells in the smooth walls absorb vitamins, minerals, and water. Condensed waste, called feces, leaves the body through the rectum.

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Last modified: May 24, 2001