CHAPTER TWENTY-THREE
The Digestive System
23.1 Overview of the Digestive System A. The digestive system consists of a muscular tube called the gastrointestinal tract (or GI tract) and a variety of accessory organs. 1. Gastrointestinal tract -- the muscular tube through which food passes. The GI tract, also known as the alimentary canal, begins with the mouth and continues through the oral cavity, pharynx, esophagus, stomach, small intestine, and large intestine which opens to the exterior through the anus. 2. Accessory organs -- along the length of the GI tract, several accessory structures produce
organic and inorganic nutrients for absorption. The primary accessory organs include: salivary glands, liver, gallbladder, and pancreas. 3. The digestive system works with other systems to support tissues that have no direct connection to the outside environment and no other means to obtain nutrients. B. Histology - The organs of the GI tract have distinctive structural and functional characteristics, but all share an underlying pattern of histological organization. There are four basic layers of the muscular tube: 1. Mucosa -- the innermost lining containing the mucous membrane of epithelial tissue supported by a basement membrane and thin layer of muscle tissue. a. Epithelium = the epithelium varies according to location within the Gl tract I. Stratified squamous epithelium - found in the oral cavity, pharynx, the upper portions of the esophagus, and rectum. ii. Simple columnar epithelium -- found within the lower portions of the esophagus, the stomach, small intestine, and most of the large intestine. Possess numerous goblet cells (mucous producing cells). b. Lamina propria = a basement membrane composed of areolar connective tissue. Contains blood vessels, sensory nerve endings, lymphatic vessels, scattered areas of Iymphoid tissue (recall Peyer's patches and MALT), and mucous glands. C. Muscularis mucosae = two concentric lavers of smooth muscle. The inner laver encircles the lumen (circular layer) and the outer layer runs parallel to the long axis of the GI tract (longitudinal layer). Contractions of the muscles within the muscularis mucosae create the folds that project from the internal surface of the GI tract such as rugae, plicae circularis, and villi. 2. Submucosa - a layer of dense irregular connective tissue surrounding the mucosa. Contains large blood vessels, lymphatic vessels, nerves, and in some regions, exocrine glands that secrete enzymes and buffers into the lumen of the GI tract. 3. Muscularis externa - dominated by smooth muscle oriented in two layers: an inner circular Iayer and an outer longitudinal layer. These layers are essential in the mechanical processing of food and the propulsion of food through the digestive tract. a. Smooth muscle cells are relatively long and slender, ranging from 5 to 10 um in diameter and 30 to 200 um in length. b. Although actin and myosin filaments are utilized in the contraction of smooth muscle, they arranged differently from that of skeletal and cardiac muscle. There are no sarcomeres or myofibrils. As a result, there are no striations in smooth muscle and is called unstriated muscl