digestive system

The digestive system - The group of organs responsible for digestion. The digestive system consists of the digestive tract (also known as the alimentary tract or canal) and various associated organs.

STRUCTURE
The mouth, pharynx (throat), oesophagus (gullet), stomach, intestines, and anus make up the digestive tract. The intestines consist of the small intestine (comprising the duodenum, jejunum, and ileum) and the large intestine (comprising the caecum, colon, and rectum).

Associated organs, such as the salivary glands, the liver, and pancreas, secrete digestive juices that help break down food as it goes through the tract.

FUNCTION
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. Chemical breakdown of food is performed by the action of enzymes (biological catalysts), 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 where it is pushed into the oesophagus and is then squeezed down into the stomach. Once in the stomach, the food is mixed with hydrochloric acid and pepsin.

These substances, produced by the stomach lining, help to 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 the fats it contains. Digestive juices released by the pancreas into the duodenum contain enzymes that further break down food.

Breakdown concludes in the small intestine, carried out by enzymes that are produced by glands in the intestinal lining. Nutrients are absorbed in the small intestine through tiny projections from the intestinal wall, called villi.

The food residue enters the large intestine, where much of the water it contains is absorbed by the lining of the colon. Undigested matter is expelled via the rectum and anus as faeces.