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Your Digestive System and How It Works

National Digestive Diseases Information Clearinghouse

The digestive system is made up of the digestive tract—a series of hollow organs joined in a long, twisting tube from the mouth to the —and other organs that U.S. Department help the body break down and absorb of Health and (see figure). Human Services Organs that make up the digestive tract NATIONAL are the mouth, , , small INSTITUTES OF HEALTH intestine, —also called the Esophagus colon—, and anus. Inside these hol­ low organs is a lining called the mucosa. In the mouth, stomach, and , the mucosa contains tiny that pro­ duce juices to help digest food. The diges­ tive tract also contains a layer of smooth Stomach muscle that helps break down food and

move it along the tract. Duodenum Two “solid” digestive organs, the liver and Descending the pancreas, produce digestive juices that Transverse Colon colon reach the intestine through small tubes Ascending colon called ducts. The gallbladder stores the Jejunum

liver’s digestive juices until they are needed Small intestine Sigmoid in the intestine. Parts of the nervous and colon

circulatory systems also play major roles in Ileum the digestive system. Cecum Rectum Why is important? Anus

When you eat —such as bread, , The digestive system. and vegetables—they are not in a form that the body can use as nourishment. Food and drink must be changed into smaller process by which food and drink are broken molecules of nutrients before they can be down into their smallest parts so the body absorbed into the and carried to can use them to build and nourish cells and cells throughout the body. Digestion is the to provide energy. How is food digested? The stomach has three mechanical tasks. First, it stores the swallowed food and liq­ Digestion involves mixing food with diges­ uid. To do this, the muscle of the upper tive juices, moving it through the digestive part of the stomach relaxes to accept large tract, and breaking down large molecules of volumes of swallowed material. The sec­ food into smaller molecules. Digestion ond job is to mix up the food, liquid, and begins in the mouth, when you chew and digestive juice produced by the stomach. swallow, and is completed in the small The lower part of the stomach mixes these intestine. materials by its muscle action. The third Movement of Food Through task of the stomach is to empty its contents the System slowly into the small intestine. The large, hollow organs of the digestive Several factors affect emptying of the stom­ tract contain a layer of muscle that enables ach, including the kind of food and the their walls to move. The movement of degree of muscle action of the emptying walls can propel food and liquid stomach and the small intestine. Carbohy­ through the system and also can mix the drates, for example, spend the least amount contents within each organ. Food moves of time in the stomach, while protein stays from one organ to the next through muscle in the stomach longer, and fats the longest. action called peristalsis. Peristalsis looks As the food dissolves into the juices from like an ocean wave traveling through the the pancreas, liver, and intestine, the con­ muscle. The muscle of the organ contracts tents of the intestine are mixed and pushed to create a narrowing and then propels the forward to allow further digestion. narrowed portion slowly down the length of Finally, the digested nutrients are absorbed the organ. These waves of narrowing push through the intestinal walls and transported the food and fluid in front of them through throughout the body. The waste products each hollow organ. of this process include undigested parts of The first major muscle movement occurs the food, known as fiber, and older cells when food or liquid is swallowed. Although that have been shed from the mucosa. you are able to start swallowing by choice, These materials are pushed into the colon, once the swallow begins, it becomes invol­ where they remain until the feces are untary and proceeds under the control of expelled by a bowel movement. the . Production of Digestive Juices Swallowed food is pushed into the esopha­ The digestive glands that act first are in the gus, which connects the throat above with mouth—the salivary glands. Saliva pro­ the stomach below. At the junction of the duced by these glands contains an esophagus and stomach, there is a ring­ that begins to digest the starch from food like muscle, called the lower esophageal into smaller molecules. An enzyme is a sphincter, closing the passage between the substance that speeds up chemical reactions two organs. As food approaches the closed in the body. sphincter, the sphincter relaxes and allows the food to pass through to the stomach.

 Your Digestive System and How It Works The next set of digestive glands is in the microvilli. These create a vast stomach lining. They produce stomach acid surface area through which nutrients can be and an enzyme that digests protein. A thick absorbed. Specialized cells allow absorbed mucus layer coats the mucosa and helps materials to cross the mucosa into the keep the acidic digestive juice from dissolv­ blood, where they are carried off in the ing the of the stomach itself. In most bloodstream to other parts of the body for people, the stomach mucosa is able to resist storage or further chemical change. This the juice, although food and other tissues of part of the process varies with different the body cannot. types of nutrients. After the stomach empties the food and Carbohydrates. The Dietary Guidelines juice mixture into the small intestine, the for Americans 2005 recommend that 45 to juices of two other digestive organs mix 65 percent of total daily calories be from with the food. One of these organs, the carbohydrates. Foods rich in carbohydrates pancreas, produces a juice that contains include bread, potatoes, dried peas and a wide array of to break down beans, rice, pasta, , and vegetables. the carbohydrate, fat, and protein in food. Many of these foods contain both starch Other enzymes that are active in the pro­ and fiber. cess come from glands in the wall of the The digestible carbohydrates—starch and intestine. sugar—are broken into simpler molecules The second organ, the liver, produces yet by enzymes in the saliva, in juice produced another digestive juice—bile. Bile is stored by the pancreas, and in the lining of the between meals in the gallbladder. At meal­ small intestine. Starch is digested in two time, it is squeezed out of the gallbladder, steps. First, an enzyme in the saliva and through the bile ducts, and into the intes­ pancreatic juice breaks the starch into mol­ tine to mix with the fat in food. The bile ecules called maltose. Then an enzyme in acids dissolve fat into the watery contents of the lining of the small intestine splits the the intestine, much like detergents that dis­ maltose into glucose molecules that can be solve grease from a frying pan. After fat is absorbed into the blood. Glucose is carried dissolved, it is digested by enzymes from the through the bloodstream to the liver, where pancreas and the lining of the intestine. it is stored or used to provide energy for the work of the body. Absorption and Transport of Nutrients Sugars are digested in one step. An enzyme in the lining of the small intestine digests Most digested molecules of food, as well as sucrose, also known as table sugar, into water and minerals, are absorbed through glucose and fructose, which are absorbed the small intestine. The mucosa of the through the intestine into the blood. Milk small intestine contains many folds that contains another type of sugar, lactose, are covered with tiny fingerlike projec­ which is changed into absorbable molecules tions called villi. In turn, the villi are cov­ by another enzyme in the intestinal lining. ered with microscopic projections called

 Your Digestive System and How It Works Fiber is undigestible and moves through the near the intestine. These small vessels carry digestive tract without being broken down the reformed fat to the of the chest, by enzymes. Many foods contain both and the blood carries the fat to storage soluble and insoluble fiber. Soluble fiber depots in different parts of the body. dissolves easily in water and takes on a soft, Vitamins. Another vital part of food that gel-like texture in the intestines. Insoluble is absorbed through the small intestine are fiber, on the other hand, passes essentially vitamins. The two types of vitamins are unchanged through the intestines. classified by the fluid in which they can be Protein. Foods such as meat, eggs, and dissolved: water-soluble vitamins (all the beans consist of giant molecules of protein B vitamins and vitamin C) and fat-soluble that must be digested by enzymes before vitamins (vitamins A, D, E, and K). Fat- they can be used to build and repair body soluble vitamins are stored in the liver and tissues. An enzyme in the juice of the fatty tissue of the body, whereas water- stomach starts the digestion of swallowed soluble vitamins are not easily stored and protein. Then in the small intestine, several excess amounts are flushed out in the . enzymes from the pancreatic juice and the Water and salt. Most of the material lining of the intestine complete the break­ absorbed through the small intestine is down of huge protein molecules into small water in which salt is dissolved. The salt molecules called amino acids. These small and water come from the food and liquid molecules can be absorbed through the you swallow and the juices secreted by the small intestine into the blood and then be many digestive glands. carried to all parts of the body to build the walls and other parts of cells. How is the digestive process Fats. Fat molecules are a rich source of energy for the body. The first step in diges­ controlled? tion of a fat such as butter is to dissolve it into the watery content of the intestine. Regulators The bile acids produced by the liver dissolve The major that control the func­ fat into tiny droplets and allow pancreatic tions of the digestive system are produced and intestinal enzymes to break the large and released by cells in the mucosa of the fat molecules into smaller ones. Some of stomach and small intestine. These hor­ these small molecules are fatty acids and mones are released into the blood of the cholesterol. The bile acids combine with digestive tract, travel back to the the fatty acids and cholesterol and help and through the , and return to the these molecules move into the cells of the digestive system where they stimulate diges­ mucosa. In these cells the small molecules tive juices and cause organ movement. are formed back into large ones, most of which pass into vessels called lymphatics

4 Your Digestive System and How It Works The main hormones that control digestion Both of these hormones work on the are gastrin, secretin, and cholecystokinin to help regulate the intake of food for (CCK): energy. Researchers are studying other hormones that may play a part in inhibiting • Gastrin causes the stomach to produce appetite, including glucagon-like peptide-1 an acid for dissolving and digesting (GPL-1), oxyntomodulin (+), and pancre­ some foods. Gastrin is also necessary atic polypeptide. for normal growth in the lining of the stomach, small intestine, and Regulators colon. Two types of nerves help control the action • Secretin causes the pancreas to send of the digestive system. out a digestive juice that is rich in Extrinsic, or outside, nerves come to the bicarbonate. The bicarbonate helps digestive organs from the brain or the neutralize the acidic stomach con­ . They release two chemicals, tents as they enter the small intestine. acetylcholine and adrenaline. Acetylcho­ Secretin also stimulates the stomach to line causes the muscle layer of the diges­ produce pepsin, an enzyme that digests tive organs to squeeze with more force protein, and stimulates the liver to pro­ and increase the “push” of food and juice duce bile. through the digestive tract. It also causes • CCK causes the pancreas to produce the stomach and pancreas to produce more the enzymes of pancreatic juice, and digestive juice. Adrenaline has the opposite causes the gallbladder to empty. It effect. It relaxes the muscle of the stom­ also promotes normal cell growth of ach and intestine and decreases the flow of the pancreas. blood to these organs, slowing or stopping digestion. Additional hormones in the digestive sys­ tem regulate appetite: The intrinsic, or inside, nerves make up a very dense network embedded in the walls • Ghrelin is produced in the stomach of the esophagus, stomach, small intestine, and upper intestine in the absence of and colon. The intrinsic nerves are trig­ food in the digestive system and stimu­ gered to act when the walls of the hollow lates appetite. organs are stretched by food. They release • Peptide YY is produced in the digestive many different substances that speed up or tract in response to a meal in the sys­ delay the movement of food and the pro­ tem and inhibits appetite. duction of juices by the digestive organs. Together, nerves, hormones, the blood, and the organs of the digestive system conduct the complex tasks of digesting and absorb­ ing nutrients from the foods and liquids you consume each day.

5 Your Digestive System and How It Works For More Information National Digestive Diseases American Dietetic Association Information Clearinghouse 120 South Riverside Plaza, Suite 2000 Chicago, IL 60606–6995 2 Information Way Phone: 1–800–877–1600 or 312–899–0040 Bethesda, MD 20892–3570 Fax: 312–899–4739 Phone: 1–800–891–5389 Email: [email protected] Fax: 703–738–4929 Internet: www.eatright.org Email: [email protected] Internet: www.digestive.niddk.nih.gov The National Digestive Diseases Informa- You may also find additional information about this topic by visiting MedlinePlus at www.medlineplus.gov. tion Clearinghouse (NDDIC) is a service of the National Institute of Diabetes and This publication may contain information about medications used to treat a health condition. When Digestive and Diseases (NIDDK). this publication was prepared, the NIDDK included The NIDDK is part of the National Insti- the most current information available. Occasion- ally, new information about medication is released. tutes of Health of the U.S. Department of For updates or for questions about any medications, Health and Human Services. Established in please contact the U.S. Food and Drug Administra- 1980, the Clearinghouse provides informa- tion at 1–888–INFO–FDA (463–6332), a toll-free call, or visit their website at www.fda.gov. Consult your tion about digestive diseases to people with doctor for more information. digestive disorders and to their families, health care professionals, and the public. The NDDIC answers inquiries, develops and distributes publications, and works closely with professional and patient organi- zations and Government agencies to coor- dinate resources about digestive diseases. Publications produced by the Clearing- house are carefully reviewed by both NIDDK scientists and outside experts.

This publication is not copyrighted. The Clearing- house encourages users of this fact sheet to duplicate and distribute as many copies as desired. This fact sheet is also available at www.digestive.niddk.nih.gov.

U.S. DEPARTMENT OF HEALTH AND HUMAN SERVICES National Institutes of Health

NIH Publication No. 08–2681 April 2008