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The digestive system consists of the digestive tract—oral cavity, esophagus, stomach, small and large intestines, and anus—and its associated glands—salivary glands, liver, and pancreas (Figure 15–1). Also called the gastrointestinal (GI) tract or alimentary canal, its function is to obtain molecules from the ingested food that are necessary for the maintenance, growth, and energy needs of the body. During digestion, proteins, complex carbohydrates, nucleic acids, and fats are broken down into their small molecule subunits that are easily absorbed through the small intestine lining. Most water and electrolytes are absorbed in the large intestine. In addition, the inner layer of the entire digestive tract forms an important protective barrier between the content of the tract’s lumen and the internal milieu of the body’s connective tissue and vasculature.


The digestive system.

The digestive system consists of the tract from the mouth (oral cavity) to the anus, as well as the digestive glands emptying into this tract, primarily the salivary glands, liver, and pancreas. These accessory digestive glands are described in Chapter 16.

Structures within the digestive tract allow the following:

  • Ingestion, or introduction of food and liquid into the oral cavity

  • Mastication, or chewing, which divides solid food into digestible pieces

  • Motility, muscular movements of materials through the tract

  • Secretion of lubricating and protective mucus, digestive enzymes, acidic and alkaline fluids, and bile

  • Hormone release for local control of motility and secretion

  • Chemical digestion or enzymatic degradation of large macromolecules in food to smaller molecules and their subunits

  • Absorption of the small molecules and water into the blood and lymph

  • Elimination of indigestible, unabsorbed components of food


All regions of the GI tract have certain structural features in common. The GI tract is a hollow tube with a lumen of variable diameter and a wall made up of four main layers: the mucosa, submucosa, muscularis, and serosa. Figure 15–2 shows a general overview of these four layers; key features of each layer are summarized here.

  • The mucosa consists of an epithelial lining; an underlying lamina propria of loose connective tissue rich in blood vessels, lymphatics, lymphocytes, smooth muscle cells, and often containing small glands; and a thin layer of smooth muscle called the muscularis mucosae separating mucosa from submucosa and allowing local movements of the mucosa. The mucosa is also frequently called a mucous membrane.

  • The submucosa contains denser connective tissue with larger blood and lymph vessels and the submucosal (Meissner) plexus of autonomic nerves. It may also contain glands and significant lymphoid tissue.

  • The thick muscularis (or muscularis externa) is composed of smooth muscle cells organized as two or more sublayers. In the internal sublayer (closer to the ...

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