The major function of the digestive system is to deliver nutrients to the body. In order to get nutrients from food into a usable form in the bloodstream, food is ingested, moved along the digestive tract, and digested into smaller fragments and molecules. Mechanical digestion occurs when food is broken into smaller pieces but the chemical composition remains unchanged. Chemical digestion occurs when food molecules are broken down into smaller molecules from breaking chemical bonds. Once digestion is complete, the nutrients are absorbed into the bloodstream and any undigested material remaining in the digestive tract is eliminated from the body.
The organs of the digestive system are categorized as part of the alimentary canal or as an accessory organ. The alimentary canal is a continuous tube that runs through the body from the mouth to the anus, with each organ that makes up the alimentary canal having a specialized function for the digestive process. The canal begins at the mouth where food is ingested, continues to the pharynx, esophagus, stomach, small intestine, large intestine, and rectum, and terminates at the anus, through which feces are eliminated. Substances inside the alimentary canal are part of the body's external environment. Only when absorption occurs does the material enter the internal environment of the body.
The walls of most of the alimentary canal include four major layers: the mucosa, submucosa, muscularis externa, and serosa.
- The mucosa is the innermost layer that lines the lumen. The lumen is the interior of the digestive tract and is composed of three sublayers: the mucous membrane, the lamina propria, and the muscularis mucosae. The mucous membrane is composed of epithelia, or surface cells called enterocytes. The lamina propria is a loose layer of tissue under the epithelium that contains small blood vessels, nerves, and lymphatic vessels. The muscularis mucosae is a thin layer of smooth muscle.
- The submucosa is the underlying layer of connective tissue that includes blood and lymphatic vessels, glands that release digestive secretions, and nerves belonging to the enteric nervous system. The enteric nervous system is composed of the many nerves that control the digestive process.
- The muscularis externa is made up primarily of smooth muscle fibers and has an inner layer of circular muscle and an outer layer of longitudinal muscle. These muscles move food through the alimentary canal and also make up regulatory sphincters which are muscular rings that allow food to move from one organ to another.
- The serosa is the outer layer of the alimentary canal made mostly of connective tissue that provides structural support.