Your Digestive System and How It Works

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 anus—and other organs that U.S. Department help the body break down and absorb food of Health and (see figure). Human Services Organs that make up the digestive tract NATIONAL are the mouth, esophagus, stomach, small INSTITUTES OF HEALTH intestine, large intestine—also called the Esophagus colon—rectum, and anus. Inside these hol low organs is a lining called the mucosa. In the mouth, stomach, and small intestine, the mucosa contains tiny glands that pro duce juices to help digest food. The diges Liver tive tract also contains a layer of smooth Stomach muscle that helps break down food and Gallbladder move it along the tract. Duodenum Two “solid” digestive organs, the liver and Pancreas 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 Appendix Rectum Why is digestion important? Anus When you eat foods—such as bread, meat, 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 blood 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 organ 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 nerves. 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 enzyme 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 structures 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 tissue 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 enzymes to break down beans, rice, pasta, fruits, 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 veins 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 urine. 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.

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