Digestive system

The group of organs responsible for digestion. It consists of the digestive tract (also known as the alimentary tract or canal) and various associated organs. The digestive tract consists of the mouth, pharynx, oesophagus, stomach, intestines, and the anus. The intestines are the small intestine (comprising the duodenum, jejunum, and ileum) and the large intestine (comprising the caecum, colon, and rectum). The associated organs, such as the salivary glands, liver, and pancreas, secrete digestive juices that break down food as it goes through the tract. Food and the products of digestion are moved from the throat to the rectum by peristalsis (waves of muscular contractions of the intestinal wall). Food is broken down into simpler substances before being absorbed into the bloodstream.Physical breakdown is performed by the teeth, which cut and chew, and the stomach, which churns the food. The chemical breakdown of food is performed by the action of enzymes, acids, and salts. Carbohydrates are broken down into simple sugars. Proteins are broken down into polypeptides, peptides, and amino acids. Fats are broken down into glycerol, glycerides, and fatty acids. In the mouth, saliva lubricates food and contains enzymes that begin to break down carbohydrates. The tongue moulds food into balls (called boli) for easy swallowing. The food then passes into the pharynx. From here, it is pushed into the oesophagus and squeezed down into the stomach, where it is mixed with hydrochloric acid and pepsin. Produced by the stomach lining, these substances help break down proteins. When the food has been converted to a semi-liquid consistency, it passes into the duodenum where bile salts and acids (produced by the liver) help to break down fats. Digestive juices released by the pancreas into the duodenum contain enzymes that further break down food. Breakdown ends in the small intestine, carried out by enzymes produced by glands in the intestinal lining. Nutrients are absorbed in the small intestine. The residue enters the large intestine, where water is absorbed. Undigested matter is expelled via the rectum and anus as faeces.