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Digestion

Animals need food to carry out all their living processes, including growth, reproduction and movement. Every cell needs to receive a constant food supply. But most food cannot be used just as it is eaten. It must be able to reach all parts of the body, and it must therefore be broken down into a form that will pass through the cell membranes. The process by which this is done is called digestion.

Even in simple, one-celled animals that have no bloodstream, such as the amoeba, food must be digested. No cell could use, say, a piece of meat or potato, even if it could be carried around the body. Digestion, then, consists of more than simply breaking the food up into small enough pieces. It involves converting it into simpler chemical substances that the cells can use.

In all but the simplest animals, digestion takes place in the alimentary canal, or digestive tract. This is basically a tube that extends from the animal's mouth, which takes in the original food, to its anus or cloaca, which expels the unwanted matter. Part of the alimentary canal is twisted and coiled, so that it is much longer than the animal itself. In man, for example, it is about 30 feet long. This gives the food plenty of chance to be digested and absorbed into the body.

There are three basic kinds of substances in food that need digestion: carbohydrates, such as sugar and starch; proteins, such as meat and fish; and fats. Chemical substances called enzymes are mixed with the food in the alimentary canal and break down these three groups in different ways. The products can all dissolve in the blood. Other parts of food - vitamins and minerals - are not broken down, because the body uses them in their natural form.

Digestion begins in the mouth. The teeth break the food into small pieces and mix it thoroughly with saliva. An enzyme in the saliva starts converting any starch in the food into sugar. After being chewed, the food is swallowed, and passes down a tube called the esophagus into the stomach. Muscles in the wall of the esophagus force the food down. The stomach produces another enzyme, which starts to break down proteins. The stomach also contains hydrochloric acid, which makes this enzyme active. A second stomach enzyme clots (solidifies) the protein in milk. This is particularly important for babies, who rely on milk for food.

After several hours, the paste-like and partially digested food enters the small intestine, where the chemical breakdown is completed. Many digestive juices containing enzymes are released there, from the walls of the intestine itself and from the pancreas. The liver produces liquid called bile, which breaks fats down into tiny droplets. Enzymes from the pancreas break down these fatty droplets as well as proteins and carbohydrates.

Tiny finger-like projections from the intestine wall absorb the digested food from the half-liquid mass. They are called villi. Blood vessels carry the food from the villi to the liver, which passes enough food to the body for its needs and stores the rest until required. The undigested food from the small intestine passes into the shorter but wider large intestine. There the undigested food loses water and becomes more solid. Finally, this waste is eliminated from the body through the anus as feces.