The Digestive System and How It Works National Digestive Diseases Information Clearinghouse What is the digestive system? The digestive system is made up of the gastrointestinal (GI) tract—also called the Mouth Esophagus digestive tract—and the liver, pancreas, and gallbladder. The GI tract is a series of hollow organs joined in a long, twisting Stomach tube from the mouth to the anus. The Liver 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 Gallbladder Pancreas 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 Small Large the body digest food. intestine intestine Bacteria in the GI tract, also called gut flora or microbiome, help with digestion. Parts of Anus Rectum the nervous and circulatory systems also play roles in the digestive process. Together, a combination of nerves, hormones, bacteria, The digestive system 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? into the blood, which then carries them throughout the body. The Dietary Guidelines Digestion is important for breaking down for Americans, 2010, recommends that 10 to food into nutrients, which the body uses 35 percent of total daily calories come from for energy, growth, and cell repair. Food protein.1 and drink must be changed into smaller molecules of nutrients before the blood Fats. Fat molecules are a rich source of absorbs them and carries them to cells energy for the body and help the body throughout the body. The body breaks absorb vitamins. Oils, such as corn, canola, down nutrients from food and drink into olive, safflower, soybean, and sunflower, are carbohydrates, protein, fats, and vitamins. examples of healthy fats. Butter, shortening, and snack foods are examples of less healthy Carbohydrates. Carbohydrates are the fats. During digestion, the body breaks down sugars, starches, and fiber found in many fat molecules into fatty acids and glycerol. foods. Carbohydrates are called simple The Dietary Guidelines for Americans, 2010, or complex, depending on their chemical recommends that 20 to 35 percent of total structure. Simple carbohydrates include daily calories come from fat.1 sugars found naturally in foods such as fruits, vegetables, milk, and milk products, as well Vitamins. Scientists classify vitamins by as sugars added during food processing. the fluid in which they dissolve. Water- Complex carbohydrates are starches and soluble vitamins include all the B vitamins fiber found in whole-grain breads and and vitamin C. Fat-soluble vitamins include cereals, starchy vegetables, and legumes. vitamins A, D, E, and K. Each vitamin has The Dietary Guidelines for Americans, 2010, a different role in the body’s growth and recommends that 45 to 65 percent of total health. The body stores fat-soluble vitamins daily calories come from carbohydrates.1 in the liver and fatty tissues, whereas the body does not easily store water-soluble Protein. Foods such as meat, eggs, and vitamins and flushes out the extra in the beans consist of large molecules of protein urine. Read more about vitamins on the that the body digests into smaller molecules Office of Dietary Supplements website at called amino acids. The body absorbs www.ods.od.nih.gov. amino acids through the small intestine 1U.S. Department of Agriculture and U.S. Department of Health and Human Services. Dietary Guidelines for Americans, 2010. 7th ed. Washington, D.C.: U.S. Government Printing Office; 2010. 2 The Digestive System and How It Works How does digestion work? delivers them to the rest of the body. Waste products of digestion pass through the large Digestion works by moving food through intestine and out of the body as a solid the GI tract. Digestion begins in the mouth matter called stool. with chewing and ends in the small intestine. As food passes through the GI tract, it Table 1 shows the parts of the digestive mixes with digestive juices, causing large process performed by each digestive molecules of food to break down into smaller organ, including movement of food, type molecules. The body then absorbs these of digestive juice used, and food particles smaller molecules through the walls of the broken down by that organ. small intestine into the bloodstream, which Table 1. The digestive process Food Particles Organ Movement Digestive Juices Used Broken Down Mouth Chewing Saliva Starches Esophagus Swallowing None None Upper muscle in stomach relaxes to let food enter and Stomach Stomach acid Protein lower muscle mixes food with digestive juice Small intestine Starches, protein, and Small intestine Peristalsis digestive juice carbohydrates Starches, fats, and Pancreas None Pancreatic juice protein Liver None Bile acids Fats 3 The Digestive System and How It Works How does food move small intestine. The muscle of the upper part of the stomach relaxes to accept large through the GI tract? volumes of swallowed material from the The large, hollow organs of the GI tract esophagus. The muscle of the lower part of contain a layer of muscle that enables their the stomach mixes the food and liquid with walls to move. The movement of organ digestive juice. walls—called peristalsis—propels food and liquid through the GI tract and mixes the Small intestine. The muscles of the small contents within each organ. Peristalsis looks intestine mix food with digestive juices like an ocean wave traveling through the from the pancreas, liver, and intestine muscle as it contracts and relaxes. and push the mixture forward to help with further digestion. The walls of the small Esophagus. When a person swallows, food intestine absorb the digested nutrients into pushes into the esophagus, the muscular tube the bloodstream. The blood delivers the that carries food and liquids from the mouth nutrients to the rest of the body. to the stomach. Once swallowing begins, it becomes involuntary and proceeds under Large intestine. The waste products of the the control of the esophagus and brain. digestive process include undigested parts The lower esophageal sphincter, a ringlike of food and older cells from the GI tract muscle at the junction of the esophagus and lining. Muscles push these waste products stomach, controls the passage of food and into the large intestine. The large intestine liquid between the esophagus and stomach. absorbs water and any remaining nutrients As food approaches the closed sphincter, the and changes the waste from liquid into stool. muscle relaxes and lets food pass through to The rectum stores stool until it pushes stool the stomach. out of the body during a bowel movement. Stomach. The stomach stores swallowed food and liquid, mixes the food and liquid with digestive juice it produces, and slowly empties its contents, called chyme, into the 4 The Digestive System and How It Works How do digestive juices in intestine, much like how detergents dissolve grease from a frying pan, so the intestinal each organ of the GI tract and pancreatic enzymes can digest the fat break down food? molecules. Digestive juices contain enzymes— Small intestine. Digestive juice produced by substances that speed up chemical reactions the small intestine combines with pancreatic in the body—that break food down into juice and bile to complete digestion. The different nutrients. body completes the breakdown of proteins, Salivary glands. Saliva produced by the and the final breakdown of starches produces salivary glands moistens food so it moves glucose molecules that absorb into the blood. more easily through the esophagus into the Bacteria in the small intestine produce stomach. Saliva also contains an enzyme that some of the enzymes needed to digest begins to break down the starches from food. carbohydrates. Glands in the stomach lining. The glands in the stomach lining produce stomach acid and What happens to the an enzyme that digests protein. digested food molecules? Pancreas. The pancreas produces a juice The small intestine absorbs most digested containing several enzymes that break down food molecules, as well as water and carbohydrates, fats, and proteins in food. minerals, and passes them on to other parts The pancreas delivers digestive juice to the of the body for storage or further chemical small intestine through small tubes called change. Specialized cells help absorbed ducts. materials cross the intestinal lining into the bloodstream. The bloodstream carries Liver. The liver produces a digestive juice simple sugars, amino acids, glycerol, and called bile. The gallbladder stores bile some vitamins and salts to the liver. The between meals. When a person eats, the lymphatic system, a network of vessels that gallbladder squeezes bile through the carry white blood cells and a fluid called bile ducts, which connect the gallbladder lymph throughout the body, absorbs fatty and liver to the small intestine. The bile acids and vitamins. mixes with the fat in food. The bile acids dissolve fat into the watery contents of the 5 The Digestive System and How It Works How is the digestive process controlled? Points to Remember Hormone and nerve regulators control the • Digestion is important for breaking digestive process. down food into nutrients, which the body uses for energy, growth, and Hormone Regulators cell repair. The cells in the lining of the stomach • Digestion works by moving food and small intestine produce and release through the gastrointestinal (GI) hormones that control the functions of the tract. digestive system. These hormones stimulate production of digestive juices and regulate • Digestion begins in the mouth appetite. with chewing and ends in the small intestine. Nerve Regulators • As food passes through the GI Two types of nerves help control the action tract, it mixes with digestive juices, of the digestive system: extrinsic and causing large molecules of food to intrinsic nerves.