The digestive system breaks down food into simple components that the body’s cells can absorb and eliminates remaining waste. Food is propelled by muscular contractions through the digestive tract, the long tube that runs from the mouth to the anus. Digestion begins in the mouth, where saliva moistens and dissolves the food and the chewing action of the teeth and tongue breaks up large particles. The process of digestion is completed when nutrients (the useful parts of food) have been absorbed into the bloodstream and waste has passed out of the anus. The body uses nutrients for energy, growth, and tissue repair.
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The digestive system consists of a long, convoluted, muscular tube or tract, which extends from the mouth to the anus, and a number of other digestive organs. These associated organs include the salivary glands, liver, and pancreas. The digestive tract is made up of a series of hollow organs, which includes the stomach and the small and large intestines.
The functions of the digestive system are to ingest food, break it down, extract the useful components, or nutrients, and dispose of the remaining waste as faeces. Food can take many hours to pass through the tract. However, the oesophagus and stomach enable us to swallow a large amount of food quickly and digest it at leisure. In an average person’s lifetime, the digestive system processes 30,000 kg (66,000 lb) of food. The digestive system can process a variety of diets, from a typical meat-based Western diet to a Japanese diet consisting mainly of rice and fish.
Most food molecules are too large to pass through cell membranes and must be broken down before they can be absorbed. Organs of the digestive system secrete juices containing enzymes (proteins that speed up chemical reactions) and acids that help to convert large molecules into small, absorbable units. Chewing physically breaks down food into small particles, exposing a large surface area to the action of enzymes. In the stomach, food is churned with digestive juices to make chyme, a semi-liquid mixture.
By the time food reaches the end of the small intestine, most large nutrient molecules have been broken down into smaller molecules. These are absorbed into the blood through tiny pores in the villi, the fronds that project from the intestinal wall. The blood carries nutrients to the liver, then to the body’s cells. Indigestible matter passes into the large intestine, where some water is absorbed before the remainder is expelled from the anus as faeces.
The nervous and hormonal systems work together to ensure that digestive juices are secreted at the right time in different parts of the digestive tract. For example, when food enters the stomach or intestines, local glands release hormones that activate the production of digestive juices. These systems also control the action of the muscular walls of the digestive tract.
From the 2010 revision of the Complete Home Medical Guide © Dorling Kindersley Limited.
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