Digestive System

Why We Need to Digest Food

The food we eat is a mixture of large, complex biomolecules -- carbohydrates, proteins and fats -- along with vitamins, minerals and water. The body cannot use these large molecules directly in the form we swallow them. Before they can be absorbed and put to use, they must be broken down, both mechanically and biochemically (with the help of enzymes), into simple substances small enough to cross the gut lining into the blood. This entire breakdown process is called digestion, and it is carried out by the digestive system.

The human digestive system has two parts working together:

• The alimentary canal -- one long, continuous tube running from the mouth to the anus.
• The digestive glands -- accessory organs that pour their secretions into this tube at different points.

The General Plan of the Canal

Starting at the front, the canal begins at the mouth, which opens into the oral (buccal) cavity. Three pairs of salivary glands -- the parotid glands near the cheek/ear and the submaxillary and sublingual glands under the jaw -- empty their secretion, saliva, into this cavity. From the mouth, food passes into the pharynx, then down the oesophagus into the stomach. The liver, gall bladder and pancreas lie alongside the stomach and duodenum and feed their secretions into the gut here. Beyond the stomach lies a long, highly coiled small intestine (made of the duodenum and the jejunum, leading on to the ileum), and beyond that the large intestine -- the ascending, transverse, descending and sigmoid parts of the colon, with a small pouch called the caecum and a finger-like vermiform appendix at the junction with the small intestine. The canal finally ends at the rectum and the anus.

Every organ named above is looked at closely in the sections that follow -- first the canal itself and its glands (structure), then how each region digests food (function), and finally how the digested nutrients are absorbed into the body.