Mouth Stomach Small intestine Large intestine

The food we eat is usually very complex (see Food: The chemicals of life) and our bodies cannot use it in this form. Most animals, including humans, have evolved a system that breaks down the complex molecules in our food into simple units that can be easily absorbed and transported throughout the body by the bloodstream.

The human digestive system is about 8 metres long. It has several main organs with specific roles contributing to the overall process of digestion.

Mouth
The teeth start digestion by physically reducing the food into smaller pieces (mechanical digestion). The canine teeth rip through flesh, the incisors bite off chunks and the molars grind and mash. This is helped by the rolling action of the tongue and the moistening effect of saliva. Mechanical digestion increases the surface area of the food so that the digestive juices can act more efficiently to break down the complex food chemicals (chemical digestion).

Saliva lubricates the food for easy swallowing, as well as containing the enzymes needed to commence starch breakdown. Enzymes are protein molecules produced by specialised glands. These "digestive juices" increase the rate of chemical reactions, with a separate enzyme required for each specific reaction.

The epiglottis is a flap which prevents swallowed food entering the trachea and lungs. Instead it continues down the oesophagus (gullet) to the stomach.

Stomach
The strong muscle walls of the stomach churn the food to make a thick "soup" (chyme). Acid and protein-digesting enzymes are added to the chyme by specialised gland cells in the stomach wall. The acid maximises the action of the protein-digesting enzymes. Mucus prevents the acid and enzymes digesting the muscles of the stomach wall (which are made of protein), although stress can reduce the effectiveness of the mucus and cause ulcers. (The "burning" sensation in the throat after vomiting is due to this acid.)

Small intestine
Food passes along the digestive tract by peristalsis, wave-like contractions of the smooth muscle in the walls.

Peristalsis

Digestion of carbohydrates and protein is completed in the first section of the small intestine (duodenum), involving enzymes from glands in the intestine walls and the pancreas. Bile from the liver (stored in the gall bladder) aids fat digestion.

By the time food reaches the second section of the small intestine (ileum), all food has been reduced to simple units (except for plant cellulose "fibre" which passes through largely undigested).These simple units are now absorbed into the bloodstream through the villi of the intestine wall (see Systems, transport and exchange). These capillary-rich folds increase the area for absorption of nutrients to about 300 square metres.

Excess sugars are stored in the liver as glycogen. The hormones insulin and glucagon regulate the uptake or release of sugar to maintain a constant sugar level in the blood regardless of the body's activities.

The circulatory system then carries the absorbed nutrients from the liver to all parts of the body.

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