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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, it functions to obtain organic molecules from food that will supply the body’s maintenance, growth, and energy requirements. During digestion proteins, complex carbohydrates, nucleic acids, and fats get broken down into their subunits which are absorbed in the small intestine and transferred into the microvasculature there. Most water and electrolytes undergo similar absorption in the large intestine. As introduced with the immune system, the mucosa of the digestive tract also constitutes a barrier with various defensive capabilities between ingested but not absorbed material and the body’s internal tissues.
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Structures within the digestive tract perform the following activities:
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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, and
Elimination of indigestible, unabsorbed components of food.
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GENERAL STRUCTURE OF THE DIGESTIVE TRACT
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All regions of the GI tract have certain structural features in common. A hollow tube with a variable diameter lumen, the tract’s wall has four main layers: the mucosa, submucosa, muscularis, and serosa. Figure 15–2 shows a general overview of these layers; key features of each layer are summarized here.
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The mucosa consists of an epithelial lining underlaid by a lamina propria of loose connective tissue containing small blood vessels, lymphatics, leukocytes, and often small glands. A thin smooth muscle layer called the muscularis mucosae separates the mucosa from the submucosa and permits local movements of the mucosa. A frequently used synonym for “mucosa” is “mucous membrane,” emphasizing the thinness of this layer.
The submucosa contains denser connective tissue with larger blood vessels, lymphatics, and the submucosal (Meissner) plexus of autonomic nerves. It may also contain glands and significant lymphoid tissue.
The thick muscularis (or muscularis externa) consists of smooth muscle cells organized as two or more sublayers. In the internal sublayer (closer to the lumen), the fiber orientation is generally circular; in the ...