Anatomy and Physiology

The digestive tract, from the esophagus to the anus, is characterized by a wall with four layers, or tunics. The layers are discussed below, from the inside lining of the tract to the outside lining:

• The mucosa is a mucous membrane that lines the inside of the digestive tract from mouth to anus. Depending on the section of the digestive tract, it protects the digestive tract wall, secretes substances, and absorbs the end products of digestion. It is composed of three layers:

  • The epithelium is the innermost layer of the mucosa. It is composed of simple columnar epithelium or stratified squamous epithelium. Also present are goblet cells and endocrine cells. Goblet cells secrete mucus that protects the epithelium from digestion, and endocrine cells secrete hormones into the blood.

  • The lamina propria lies outside the epithelium. It is composed of areolar connective tissue. Blood vessels and lymphatic vessels present in this layer provide nutrients to the epithelial layer, distribute hormones produced in the epithelium, and absorb end products of digestion from the lumen. The lamina propria also contains the mucosa‐associated lymphoid tissue (MALT), nodules of lymphatic tissue bearing lymphocytes and macrophages that protect the GI tract wall from bacteria and other pathogens that may be mixed with food.

  • The muscularis mucosae, the outer layer of the mucosa, is a thin layer of smooth muscle responsible for generating local movements. In the stomach and small intestine, the smooth muscle generates folds that increase the absorptive surface area of the mucosa.

• The submucosa lies outside the mucosa. It consists of areolar connective tissue containing blood vessels, lymphatic vessels, and nerve fibers.

• The muscularis (muscularis externa) is a layer of muscle. In the mouth and pharynx, it consists of skeletal muscle that aids in swallowing. In the rest of the digestive tract, it consists of smooth muscle (three layers in the stomach, two layers in the small and large intestines) and associated nerve fibers. The smooth muscle is responsible for movement of food by peristalsis and mechanical digestion by segmentation. In some regions, the circular layer of smooth muscle enlarges to form sphincters, circular muscles that control the opening and closing of the lumen (such as between the stomach and small intestine).

• The serosa is a serous membrane that covers the muscularis externa of the digestive tract in the peritoneal cavity. The following is a description of the various types of serosae associated with the digestive system:

  • The adventitia is the serous membrane that lines the muscularis externa of the oral cavity, pharynx, esophagus, and rectum.

  • The visceral peritoneum is the serous membrane that lines the stomach, large intestine, and small intestine.

  • The mesentery is an extension of the visceral peritoneum that attaches the small intestine to the rear abdominal wall.

  • The mesocolon is an extension of the visceral peritoneum that attaches the large intestine to the rear of the abdominal wall.

  • The parietal peritoneum lines the abdominopelvic cavity (abdominal and pelvic cavities). The abdominal cavity contains the stomach, small intestine, large intestine, liver, spleen, and pancreas. The pelvic cavity contains the urinary bladder, rectum, and internal reproductive organs.