What is digestion?
Food and drink must be changed into smaller molecules of nutrients to be absorbed into the blood and carried to cells throughout the body. Digestion is the process by which food and drink are broken down into smaller parts so that the body can use them to build and nourish cells, and to provide energy.

How does the digestive process work?
Digestion involves:

• the mixing of food
• the movement of food through the digestive tract
• a chemical breakdown of large molecules of food into smaller molecules

Digestion begins in the mouth, where food and drink is taken in, and is completed in the small intestine.

What is included in the digestive system?
The digestive system is made up of the digestive tract and other organs that aid in digestion.

The digestive tract is a series of hollow organs joined in a long, twisting tube from the mouth to the anus, consisting of the following:

• mouth
• esophagus
• stomach
• small intestine
• large intestine
• rectum
• anus

Organs that help with digestion, but are not part of the digestive tract, include the following:

• tongue
• glands in the mouth that make saliva
• pancreas
• liver
• gallbladder

Parts of other organ systems, like nerves and blood, also play a major role in the digestive process.

How does food move through the digestive system?
In a wave-like movement called peristalsis, muscles propel food and liquid along the digestive tract. In general, there are six steps in the process of moving food and liquid through the digestive system:

1. The first major muscle movement is swallowing food or liquid. The start of swallowing is voluntary, but once it begins, the process becomes involuntary and continues under the control of the nerves.

2. The esophagus, which connects the throat above with the stomach below, is the first organ into which the swallowed food goes.

3. Where the esophagus and stomach join, there is a ringlike valve that closes the passage between the two organs. When food nears the closed ring, the surrounding muscles relax and allow the food to pass into the stomach, and then it closes again.

4. The food then enters the stomach, which completes three mechanical tasks: stores, mixes, and empties.

First, the stomach stores the swallowed food and liquid, which requires the muscle of the upper part of the stomach to relax and accept large volumes of swallowed material.

Second, the lower part of the stomach mixes up the food, liquid, and digestive juices produced by the stomach by muscle action.

Third, the stomach empties the contents into the small intestine.

5. The food is digested in the small intestine and dissolved by the juices from the pancreas, liver, and intestine, and the contents of the intestine are mixed and pushed forward to allow further digestion.

6. Last, the digested nutrients are absorbed through the intestinal walls. The waste products, including undigested parts of the food, known as fiber, and older cells that have been shed from the mucosa, move into the colon. Waste products usually in the colon remain for a day or two until the feces are expelled by a bowel movement.

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