The adult gastrointestinal tract consists of a continuous fibromuscular tube that extends from the mouth to the anus. The tract is in contact with the external environment at both ends.

The gastrointestinal system consists of the digestive tract (mouth, oesophagus, stomach and intestines) in association with the accessory digestive glands (salivary glands, pancreas and biliary system). The overall function of the digestive system is to transfer the nutrients in food from the external environment to the internal environment. Once in the body, the nutrients can be distributed to the cells of the body via the circulation. The waste material of digestion is also excreted via the circulation. Nutrients, water and salts are absorbed from digested food and all products that cannot be absorbed are retained in the digestive tract until they are eliminated. The gastrointestinal tract is regulated by both the autonomic nervous system and hormonal mechanisms, which act in conjunction with a variety of gastrointestinal peptides (hormones, neurocrines or paracrines).

In this chapter the general principles and the basic mechanisms involved in the overall function of the digestive system will be examined. Figure 2.1 illustrates both the component organs of the gastrointestinal tract and the accessory organs that are required for the digestive system to function.

Figure 2.1 Component organs and accessory organs of the gastrointestinal tract. Reproduced with permission from Watson (2000).

Box 2.1 Generalised layers of the gastrointestinal tract.

• muscularis

• serosa (fibrous outer layer)

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