The digestive system is made up of the gastrointestinal (GI) tract—also called the digestive tract—and the liver, pancreas, and gallbladder. The GI tract is a series of hollow organs joined in a long, twisting tube from the mouth to the anus. The hollow organs that make up the GI tract are the mouth, esophagus, stomach, small intestine, large intestine—which includes the rectum—and anus. Food enters the mouth and passes to the anus through the hollow organs of the GI tract. The liver, pancreas, and gallbladder are the solid organs of the digestive system. The digestive system helps the body digest food.
Bacteria in the GI tract, also called gut flora or microbiome, help with digestion. Parts of the nervous and circulatory systems also play roles in the digestive process. Together, a combination of nerves, hormones, bacteria, blood, and the organs of the digestive system completes the complex task of digesting the foods and liquids a person consumes each day.
Why is digestion important?
Digestion is important for breaking down food into nutrients, which the body uses for energy, growth, and cell repair. Food and drink must be changed into smaller molecules of nutrients before the blood absorbs them and carries them to cells throughout the body. The body breaks down nutrients from food and drink into carbohydrates, protein, fats, and vitamins.
Carbohydrates. Carbohydrates are the sugars, starches, and fiber found in many foods. Carbohydrates are called simple or complex, depending on their chemical structure. Simple carbohydrates include sugars found naturally in foods such as fruits, vegetables, milk, and milk products, as well as sugars added during food processing. Complex carbohydrates are starches and fiber found in whole-grain breads and cereals, starchy vegetables, and legumes. The
Dietary Guidelines for Americans, 2010, recommends that 45 to 65 percent of total daily calories come from carbohydrates.
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Protein. Foods such as meat, eggs, and beans consist of large molecules of protein that the body digests into smaller molecules called amino acids. The body absorbs amino acids through the small intestine into the blood, which then carries them throughout the body. The
Dietary Guidelines for Americans, 2010, recommends that 10 to 35 percent of total daily calories come from protein.
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Fats. Fat molecules are a rich source of energy for the body and help the body absorb vitamins. Oils, such as corn, canola, olive, safflower, soybean, and sunflower, are examples of healthy fats. Butter, shortening, and snack foods are examples of less healthy fats. During digestion, the body breaks down fat molecules into fatty acids and glycerol. The
Dietary Guidelines for Americans, 2010, recommends that 20 to 35 percent of total daily calories come from fat.
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Vitamins. Scientists classify vitamins by the fluid in which they dissolve. Water-soluble vitamins include all the B vitamins and vitamin C. Fat-soluble vitamins include vitamins A, D, E, and K. Each vitamin has a different role in the body’s growth and health. The body stores fat-soluble vitamins in the liver and fatty tissues, whereas the body does not easily store water-soluble vitamins and flushes out the extra in the urine.
Digestion works by moving food through the GI tract. Digestion begins in the mouth with chewing and ends in the small intestine. As food passes through the GI tract, it mixes with digestive juices, causing large molecules of food to break down into smaller molecules. The body then absorbs these smaller molecules through the walls of the small intestine into the bloodstream, which delivers them to the rest of the body. Waste products of digestion pass through the large intestine and out of the body as a solid matter called stool.
Table 1 shows the parts of the digestive process performed by each digestive organ, including movement of food, type of digestive juice used, and food particles broken down by that organ.
Table 1. The digestive process
| Organ | Movement | Digestive Juices Used | Food Particles
Broken Down |
|---|
| Mouth | Chewing | Saliva | Starches |
| Esophagus | Swallowing | None | None |
| Stomach | Upper muscle in stomach relaxes to let food enter and lower muscle mixes food with digestive juice | Stomach acid | Protein |
| Small intestine | Peristalsis | Small intestine
digestive juice | Starches, protein, and
carbohydrates |
| Pancreas | None | Pancreatic juice | Starches, fats, and
protein |
| Liver | None | Bile acids | Fats |
