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Digestive System The digestive system is a group of organs responsible for the conversion of food into nutrients and energy needed by the body. In humans, the digestive system consists of the mouth, esophagus, stomach, and small and large intestines. Several glands—salivary glands, liver, gall bladder, and pancreas— secrete digestive juices containing enzymes that break down foods chemically into smaller molecules that are more easily absorbed by the body. The digestive system also separates and disposes of waste products ingested with the food. The digestive tube made up of these organs is known as the alimentary canal. Ingestion Food begins its path through the digestive system in the mouth, where the teeth break it into smaller pieces through the processes of biting and chewing. The tongue rolls these pieces into balls. The sight, taste, and smell of the food cause the salivary glands to produce more saliva, the liquid that always coats the mouth. An enzyme in the saliva called amylase begins the breakdown of carbohydrates into simple sugars. A ball of chewed food mixed with saliva is called a bolus. The process of swallowing involves moving a bolus to the pharynx at the back of the mouth. In the pharynx, rings of muscles force the food into the esophagus, the first part of the upper digestive tube. The esophagus extends from the bottom part of the throat to the upper part of the stomach. The esophagus does not take part in digestion. Its job is to move the bolus into the stomach. Food is moved through the esophagus and other parts of the alimentary canal by a wavelike muscular motion called peristalsis (pronounced pear-i-STALL-sis). This motion consists of the alternate contraction and relaxation of the smooth muscles lining the tract. At the junction of the esophagus and stomach sits a powerful muscle, the esophageal sphincter, which acts as a valve to keep food and stomach acids from flowing back into the esophagus and mouth. Digestion in the Stomach Words to Know Alimentary canal Tube formed by the pharynx, esophagus, stomach, and intestines through which food passes. Bile Bitter, greenish liquid produced in the liver and stored in the gall bladder that dissolves fats. Carbohydrate A molecule made up of carbon, hydrogen, and oxygen atoms that functions as food, as energy storage, and as structural molecules of living organisms. Enzyme A molecule produced by living cells that begins and maintains a biochemical reaction. Peristalsis Wavelike motion of the digestive system that moves food through the alimentary canal. Protein A large molecule built of amino acids that is essential to the structure and functioning of all living cells. Sphincter A circular muscle that controls the opening of a bodily passage. Chemical digestion begins in the stomach. The stomach is a large, hollow, pouch-shaped muscular organ. Food in the stomach is broken down by the action of gastric juice, which contains hydrochloric acid and pepsin, an enzyme that digests protein. Gastric juice is secreted from the linings of the stomach walls, along with mucus that helps to protect the stomach lining from the action of the acid. Three layers of powerful stomach muscles churn food into a thick liquid called chyme (pronounced KIME). From time to time, chyme is passed through the pyloric sphincter, the opening between the stomach and the small intestine. Digestion and Absorption in the Small Intestine The small intestine is a long, narrow tube running from the stomach to the large intestine. The small intestine is greatly coiled and twisted. Its full length is about 20 feet. The small intestine is subdivided into three sections: the duodenum (pronounced do-o-DEE-num), the jejunum (pronounced jeJOO-num), and the ileum (pronounced ILL-ee-um). The duodenum is about 10 inches long and connects with the lower portion of the stomach. When chyme reaches the duodenum, it is further broken down by intestinal juices and through the action of the pancreas and gall bladder. The pancreas is a large gland located below the stomach that secretes pancreatic juice into the duodenum through the pancreatic duct. There are three enzymes in pancreatic juice that break down carbohydrates, fats, and proteins. The gall bladder, located next to the liver, stores bile produced by the liver. While bile does not contain enzymes, it contains bile salts that help to dissolve fats. The gall bladder empties bile into the duodenum when chyme enters that portion of the intestine. The jejunum is about 8 feet long. The digested carbohydrates, fats, proteins, and most of the vitamins and minerals are absorbed in this section. The inner lining of the small intestine is composed of up to five million tiny, fingerlike projections called villi. The villi increase the rate of absorption of nutrients into the bloodstream by greatly increasing the surface area of the small intestine. The ileum, the last section of the small intestine, is the longest section of the digestive system, measuring about 11 feet. It absorbs nutrients into the bloodstream. Absorption and Elimination in the Large Intestine The large intestine is wider and heavier than the small intestine. It is also much shorter, only about 5 feet long. It rises up on the right side of the body, crosses over to the other side underneath the stomach, descends on the left side, then forms an S shape before reaching the rectum and anus. The muscular rectum, about 6 inches long, expels feces through the anus, which has a large muscular sphincter that controls the passage of waste matter. The large intestine removes water from the waste products of digestion and returns some of it to the bloodstream. Fecal matter contains undigested food, bacteria, and cells from the walls of the digestive tract. Millions of bacteria in the large intestine help to produce certain B vitamins and vitamin K. These vitamins are absorbed into the bloodstream along with the water. "Digestive System." UXL Encyclopedia of Science, edited by Amy Hackney Blackwell and Elizabeth Manar, 3rd ed., UXL, 2015. Research in Context, go.galegroup.com/ps/i.do?p=MSIC&sw=w&u=pioneer&v=2.1&id=GALE%7CCV2644300344&it=r&asid=104473f919bdfe9146c4adab 8c574824. Accessed 24 Feb. 2017.