The digestive system consists of the alimentary canal and its principal associated organs, namely, the tongue, teeth, salivary glands, pancreas, liver, and gallbladder.
The lumen of the alimentary canal is physically and functionally external to the body
As it passes through the alimentary canal, food is broken down physically and chemically so that the degraded products can be absorbed into the body. The various segments of the alimentary canal are morphologically specialized for specific aspects of digestion and absorption.
After preliminary maceration, moistening, and formation into a bolus by the actions of the structures of the oral cavity and salivary glands, food passes rapidly through the pharynx to the esophagus. The rapid passage of food through the pharynx keeps it clear for the passage of air. The food passes more slowly through the gastrointestinal tract, and during its transit through the stomach and small intestine, the major alterations associated with digestion, solubilization, and absorption occur. Absorption occurs chiefly through the wall of the small intestine. Undigested food and other substances within the alimentary canal, such as mucus, bacteria, desquamated cells, and bile pigments, are excreted as feces.
The alimentary mucosa is the surface across which most substances enter the body
The alimentary mucosa performs numerous functions in its role as an interface between the body and the environment. These include
® Secretion. The lining of the alimentary canal secretes, at specific sites, digestive enzymes, hydrochloric acid, mucin, and antibodies.
• Absorption. The epithelium of the mucosa absorbs metabolic substrates, e.g., the breakdown products of digestion, as well as vitamins, water, electrolytes, recyclable materials such as bile components and cholesterol, and other substances essential to the functions of the body.
• Barrier. The mucosa serves as a barrier to prevent the entry of noxious substances, antigens, and pathogenic organisms.
• Immunologic protection. Lymphatic tissue within the mucosa serves as the body's first line of immune defense.
The functions listed above are discussed at the beginning of the next chapter. The digestive system is considered in three chapters that deal, respectively, with the oral cavity and pharynx (this chapter), the esophagus and gastrointestinal tract (Chapter 16), and the liver, gallbladder, and pancreas (Chapter 17).
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