Your digestive system starts with your mouth, where food enters and is broken down for processing. When you swallow, food passes through a narrow tube called the esophagus, which has muscles that squeeze food particles down toward the stomach.
The Digestive System
Saliva secreted inside your mouth when you eat begins the digestive process of breaking down the food.Waves of muscular contractions move the food through the digestive tract—
the esophagus, stomach, small intestine, colon, and rectum—to the anus, from which it is eliminated from the body.Along the way, other organs of the digestive system—the liver, gallbladder, and pancreas—secrete juices that promote digestion.The appendix, attached to the large intestine, has no known function.
The adult stomach has a capacity of 2.5 to 3 pints. When food enters the stom- ach it is held, churned, and released slowly into the small intestine at the rate required for proper digestion. The rate of release depends on the type of food. Carbohydrate-rich food passes through the stomach in a few hours, while foods high in protein and fat are retained much longer. The rate at which the stomach contracts and pushes food on to the small intestine is controlled by nerves and hormones released by the stomach, small intestine, and other digestive organs. A muscular valve called the pylorus keeps food in the stomach and prevents back- ﬂow from the small intestine into the stomach. Food held in the stomach is bathed in acidic gastric juices that begin breaking down proteins and killing bacteria.
The small intestine is a 21-foot coiled tube in the abdomen that is divided into three parts—the duodenum, the jejunum, and the ileum. The ﬁrst foot or so is the duodenum, which receives food from the stomach. The duodenum releases hormones to ensure proper digestion of the food based on the amounts of carbo- hydrate, protein, and fat the food contains. Digestive juices from the pancreas and gallbladder enter the duodenum through the common bile duct. In the duodenum, iron, calcium, and folic acid are absorbed. All food looks the same once it has passed through the duodenum. This watery mixture of partially digested food, fat globules (known as micelles), enzymes, and other secretions is called chyme.
The next 8 feet of small intestine is the jejunum. Most of the digestion and absorption of water and nutrients occurs in this portion of the small intestine. Bile salts (used in breaking down fat for digestion) and vitamin B12 are absorbed in the ileum, which makes up the remaining 12 feet of small intestine.
Just as the lungs use air sacs to increase their surface area for gas exchange, so the small intestine uses villi (tiny, ﬁngerlike projections) to ensure efﬁcient nutri- ent absorption. The villi are further made up of microvilli, which provide a brush border with folds upon folds of membranes through which absorption of nutri- ents can take place. The total surface area of the small intestine is approximately 350 square yards, or 200 times the surface area of the skin. Each villus contains blood and lymph vessels to transport absorbed nutrients for use throughout the body.
When the chyme passes from the small intestine into the large intestine (colon) through the ileocecal valve (a muscle that prevents backﬂow of large- intestine contents into the small intestine), it is a watery mix of ﬁbrous waste, indigestible carbohydrates (cellulose), some electrolytes (such as sodium and potassium), and other by-products of digestion. The 6-foot-long colon absorbs most of the remaining liquid and electrolytes. Harmless bacteria in the colon digest fermented, unabsorbed carbohydrates and produce gas as a by-product. These bacteria make up about 30 percent of the solid component of stool.
The cecum (a blind pouch) and its attached appendix (a narrow, worm-shaped tube) lie just below the junction of the small and large intestines. The large intestine is made up of four sections. The ﬁrst three sections are the ascending colon (which connects with the ileum and ascends up the right side of the abdomen), the transverse colon (the longest portion, which stretches across the abdomen), and the descending colon (which descends along the left side of the abdomen). The sigmoid colon, the fourth and narrowest section of the large intestine, links the descending colon and the rectum. The rectum, about 8 inches in length, can expand considerably to permit storage of stool until it is expelled through the anal canal.
The digestive system uses both hormones (chemical messengers) and nerves to control digestion. The digestive nervous system, with more than 100 million nerve cells, is as large and as complex as the spinal cord. All of the more than 30 chemicals used to transmit instructions in the brain are found in the digestive nervous system. The digestive nervous system communicates with the brain through the vagus nerve, with messages from the digestive tract outnumbering those from the brain by about nine to one.