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The digestive system consists of the gastrointestinal (GI) tract (or gut), which includes the oral cavity (mouth), esophagus, stomach, small intestine, and large intestine, and the accessory organs, which include the salivary glands, pancreas, liver, and gallbladder. The major function of the digestive system is to acquire nutrient molecules from ingested food and eliminate indigestible remnants. The digestive system also maintains a barrier between the contents of its lumen and body tissues and provides immune protection against infectious disease. The activities of the digestive system are regulated by the enteric nervous system and by both local and systemic hormones.


The digestive system consists of all organs of the GI tract and associated organs that participate in the process of digestion—the breakdown of ingested food into its nutrient molecules, the absorption of those molecules into the blood and lymphatic capillaries, and the elimination of waste products (Figure 12-1A).

Figure 12-1:

Location and histologic organization of the organs of the digestive system. A. Location of the organs of the gastrointestinal (GI) tract and accessory digestive organs. B. The figure on the left shows the overall organization of the wall of the organs of the gastrointestinal tract. The three panels on the right compare the arrangement of epithelium in the stomach (pits and glands), small intestine (villi and glands), and colon (glands only).


The GI tract is a continuous hollow muscular tube structurally modified into discrete regions and organs that carry out specific functions, as follows:

  • Oral cavity (mouth). Contains the tongue, teeth, and minor salivary glands. The ducts of the major salivary glands open in the oral cavity, where the process of digestion is initiated.

  • Pharynx and esophagus. Transport food from the mouth to the stomach.

  • Stomach. Digests food and secretes hormones.

  • Small intestine (duodenum, jejunum, ileum). Completes the digestive process and absorbs nutrient molecules.

  • Large intestine (cecum, colon, rectum, anus). Absorbs water and electrolytes and compacts and eliminates feces.


The following accessory organs are located outside of the GI tract proper and deliver their secretions, which aid digestion, to the gut via long ducts:

  • Salivary glands (parotid, submandibular, sublingual). Produce saliva.

  • Pancreas. Produces digestive enzymes, which act in the small intestine, and hormones, which are important for glucose and lipid metabolism.

  • Liver. Removes and secretes substances into the blood and produces bile.

  • Gallbladder. Concentrates and stores bile.


The wall of the GI tract distal to the pharynx consists of four concentric layers: the mucosa, submucosa, muscularis externa, and serosa or adventitia. ...

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