Diseases of the Digestive System - Digestive functions and organs
T he function of the digestive system is to accept food and water through the mouth, to break down the food's chemical structure so that its nutrients can be absorbed into the body, a process called digestion , and to expel undigested particles. This process takes place as the food passes through the entire alimentary tract . This tract, also called the gastrointestinal tract , is a long, hollow passageway that begins at the mouth and continues on through the esophagus, the stomach, the small intestine, the large intestine, the rectum, and the anus. The salivary glands, the stomach glands, the liver, the gallbladder, and the pancreas release substances into the gastrointestinal tract that help the digestion of various food substances.
Digestion begins in the mouth where food is shredded by chewing and mixed with saliva, which helps break down starch into sugars and lubricates the food so that it can be swallowed easily. The food then enters the esophagus , a muscular tube that forcibly squeezes the food down toward the stomach, past the cardiac sphincter , a ring of muscle at the entrance of the stomach that opens to allow food into the stomach.
The stomach acts as a reservoir for food, churns the food, mixes it with gastric juices, and gradually releases the food into the small intestine. Some water, alcohol, and glucose are absorbed directly through the stomach into the bloodstream. Enzymes secreted by the stomach help break down proteins and fats into simpler substances. Hydrochloric acid secreted by the stomach kills bacteria and prepares some minerals for absorption in the small intestine. Some food may leave the stomach one minute after it enters, while other parts of a meal may remain in the stomach for as long as five hours.
The food passes from the stomach to the first section of the small intestine, the duodenum , where it is acted on by pancreatic enzymes that help break down fats, starches, proteins, and other substances. While the food is in the duodenum it is also digested by bile , which is produced by the liver and stored in the gallbladder. During a meal, the gallbladder discharges its bile into the duodenum. The bile promotes the absorption of fats and vitamins.
The semidigested food is squeezed down the entire length of the intestines by a wavelike motion of the intestinal muscles called peristalsis . Digestion is largely completed as the food passes through 20 feet of small intestine, which absorbs the digested food substances and water and passes them into the bloodstream. The food nutrients are distributed by the bloodstream throughout the body and used by the body cells.
Those parts of the food that are indigestible, such as the skins of fruits, pass into the large intestine, or colon , along with bacteria, bile, some minerals, various cells, and mucus. This combination of substances makes up the feces , which are stored in the colon until defecation . Some water and salts in the feces are absorbed through the walls of the colon into the bloodstream. This conserves the body's fluids and dries the feces. The formation of a semisolid fecal mass helps precipitate defecation.