Illustration from The National Digestive Diseases Information Clearinghouse 
The digestive system is a series of hollow organs joined in a long, twisting tube from the mouth to the anus. Inside this tube is a lining called the mucosa. In the mouth, stomach, and small intestine, the mucosa contains tiny glands that produce juices to help digest food. There are also two solid digestive organs, the liver and the pancreas, which produce juices that reach the intestine through small tubes. In addition, nerves and blood play a major role in the digestive system.
Food, as it is eaten, is not in a form that the body can use as nourishment. It must be changed into smaller molecules of nutrients that can be absorbed into the blood and carried to cells throughout the body. Digestion is the process by which food is broken down into its smallest parts so that the body can use it to build and nourish cells and to provide energy. Digestion involves the mixing of food, its movement through the digestive tract, and the chemical breakdown of the large molecules of food into smaller molecules. Digestion begins in the mouth, with chewing and swallowing.
Two types of nerves help to control the action of the digestive system. Extrinsic (outside) nerves come to the digestive organs from the brain or from the spinal cord, and they trigger the release of the chemicals acetylcholine and adrenaline. Acetylcholine causes the muscles of the digestive organs to squeeze with more force, and increases the transit speed of matter through the digestive tract. Adrenaline relaxes the muscle of the stomach and intestine and decreases the flow of blood to these organs.
Intrinsic (inside) nerves, which make up a very dense network embedded in the walls of the esophagus, stomach, small intestine, and colon, are triggered to act when the walls of the hollow digestive organs are stretched by food. In this regard they are rather like the strings of a musical instrument, which will play different notes depending on their tension. The varying tension levels of the intrinsic nerves trigger similarly varying reactions. Instead of sounding different notes they release many different substances, and either speed up or delay the movement of food and the production of juices by the digestive organs. As you can guess, the key roles of these extrinsic and intrinsic nerves in the GI tract help explain why stress has such a powerful effect on the digestive tract, and thus IBS. (I'll have more to say about this in Day 5.)
The hollow organs of the digestive system, such as the stomach and colon, contain muscles that enable their walls to move. The movement of organ walls can propel food and liquid and also can mix the contents within each organ. Typical movement of the esophagus, stomach, and intestine is called peristalsis. The action of peristalsis looks like an ocean wave moving through the muscle. The muscle of the organ produces a narrowing and then propels the narrowed portion slowly down the length of the organ. These waves of narrowing push the food and fluid in front of them through each hollow organ.
The first major muscle movement occurs when food or liquid is swallowed. Although beginning to swallow is a voluntary action, once the swallow begins it becomes involuntary and proceeds under control of the nerves. The esophagus is the organ into which the swallowed food is pushed. It connects the throat above with the stomach below. At the junction of the esophagus and stomach, there is a ring-like muscle that closes the passage between the two organs. However, as the food approaches the closed ring, the muscle relaxes and allows the food to pass. The food then enters the stomach, which has three tasks to do. First, the stomach must store the swallowed food and liquid. This requires the muscles of the upper part of the stomach to relax and accept large volumes of swallowed material. The second job is to mix the food and liquid with digestive juices, and this is what the lower part of the stomach does. The third task of the stomach is to empty its contents slowly into the small intestine.
Several factors affect emptying of the stomach, including the nature of the food (mainly its fat and protein content) and the degree of muscle action of the emptying stomach and the next organ to receive the stomach contents (the small intestine). As the food is digested in the small intestine, the contents of the intestine are mixed and pushed forward in the GI tract.
Finally, almost all of the digested nutrients are absorbed through the intestinal walls. The waste products of this process are undigested parts of the food, including soluble and insoluble fibers, and older cells that have been shed from the mucosa. These materials are propelled into the colon, which extracts any available nutritional material the small intestine was unable to collect. The colon is also where the reabsorption of water, electrolytes, and bile salts occurs, and contents are solidified into solid waste. The colon stores the waste products in the form of feces, usually for a day or two. If the waste products move too slowly through the colon, too much water is absorbed and the stool becomes hard and dry, with constipation resulting. If the material passes through the colon too quickly, not enough water is absorbed and diarrhea results.
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 The National Digestive Diseases Information Clearinghouse (NDDIC) is a service of the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK). The NIDDK is part of the National Institutes of Health under the U.S. Public Health Service. Established in 1980, the clearinghouse provides information about digestive diseases to people with digestive disorders and to their families, health care professionals, and the public. NDDIC answers inquiries; develops, reviews, and distributes publications; and works closely with professional and patient organizations and Government agencies to coordinate resources about digestive diseases.
All content is copyrighted by Heather Van Vorous and MAY NOT BE REPRODUCED without permission.