Alimentary Canal

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Chapter: Anatomy and Physiology for Health Professionals: Digestive System

The two major divisions of the digestive system are the alimentary canal and the accessory digestive organs.

Alimentary Canal

Alimentary Canal

The two major divisions of the digestive system are the alimentary canal and the accessory digestive organs. The digestive system extends from the mouth to the anus, and includes the mouth, pharynx, esophagus, stomach, small intestine, large intestine, rectum, and anus (FIGURE 24-1). Its associated accessory organs include the salivary glands, liver, gallbladder, and pancreas. The secretions from these accessory organs empty via ducts into the digestive tract. Glandular organ secretions are made up of water, enzymes, buf-fers, and other components. These secretions assist in preparing organic and inorganic nutrients for absorp-tion across the epithelium of the digestive tract. The digestive system is basically a tube open at both ends that supplies nutrients for body cells. Its surface area in an adult is 186 square meters.


The alimentary canal is an 8 meter long continu-ous muscular tube passing through the thoracic and abdominopelvic cavities (FIGURE 24-2). This canal is also known as the gastrointestinal tract or gut. In a dead body, the alimentary canal is longer, totaling 9 m, because it has lost its muscle tone.



Histology of the Alimentary Canal

The digestive tube consists of four major layers, pres-ent from the esophagus to the anus (FIGURE 24-3): an internal mucosa and an external serosa, with a sub-mucosa and a muscularis in between. Each layer has a dominant type of tissue, with a certain role in the breakdown of food.


Mucosa

In the alimentary canal, the primary functions of the mucosa are to secrete, absorb, and protect. It secretes digestive enzymes, hormones, and mucus; absorbs the final products of digestion into the blood; and protects against infections. The mucosa (mucous membrane), which is more complex than the majority of other mucosae in the body, consists of three layers:

 The inner mucous epithelium, which is made up of moist stratified squamous epithelium in the mouth, oropharynx, esophagus, and anal canal; in other areas, it is a simple columnar epithelium with many mucus-secreting cells— this mucus protects some digestive organs from being digested by their own enzymes and assists with passing food through the tract; in the stomach and small intestine, the mucosa also has enzyme-synthesis and hormone-secretion cells—here the mucosa is partially an endocrine organ as well as a digestive organ

 A loose connective tissue called the lamina propria; it is made up of loose areolar connective tissue and its capillaries absorb digested nutrients as well as nourish the epithelium; its lymphoid follicles, part of the mucosa-associated lymphoid tissue or MALT, help defend against pathogens such as bacteria, which can easily access the digestive tract; larger amounts of lymphoid folli-cles exist in the tonsils of the pharynx and in the appendix

 A thin outer layer of smooth muscle (muscularis­mucosae); it has projections extending into the lumen that increase its absorptive surface­.

Submucosa

The submucosa is made up of loose areolar connec-tive tissue with glands, blood vessels, lymphatic ves-sels, lymphoid follicles, and nerve fibers. It nourishes surrounding gastrointestinal tract wall tissues and carries away absorbed materials. The submucosa has many elastic fibers, which allow the stomach to return to its normal shape after it has temporarily held a large amount of food.

Muscularis Externa

The muscularis externa produces movements of the tube and is made of two smooth muscle tissue coats. Circular fibers of the inner coat encircle the tube, causing contraction, and longitudinal fibers run lengthwise, causing shortening of the tube. Cer-tain areas of the tract have a thickened circular layer, which forms sphincters that act as valves, controlling passage of food and preventing backflow. This layer is also referred to simply as the muscularis. It functions in segmentation and peristalsis.

Serosa

The serosa is the outermost layer of the intraper-itoneal organs and is also known as the visceral peritoneum. In most GI tract organs, it is made up of areolar connective tissue covered with mesothe-lium, which is a single layer of squamous epithelial cells. In the esophagus, the serosa is replaced by an adventitia, an ordinary dense connective tissue binding the esophagus to the surrounding struc-tures.The retroperitoneal organs have an adventitia facing the dorsal body wall and a serosa facing the peritoneal cavity.

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