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Digestive System and GI Tract-2009 - 1 Digestive System and...

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1 Digestive System and GI Tract – 2010 The digestive tract is basically a muscular tube about 30 feet in length that extends from the lips to the anus. It is the site for ingestion, digestion, absorption, and elimination of the food we take into our mouths. Associated with this muscular tube are several accessory digestive organs that include salivary glands, tongue, teeth, gall bladder, liver and pancreas. These so called accessory organs are vital. The muscular tube begins with the mouth which leads to the esophagus which drains into the stomach. The stomach empties into the small intestine which consists of three parts: the duodenum, the jejunum, and the ileum. The ileum drains into the large intestine through the ileocecal valve. The colon leads to the rectum and thence to the anus. Refer to the attached basic diagram of the muscular tube. The tube consists of four concentric layers, sometimes referred to as tunicae (sing. tunica). Beginning from the luminal surface, these layers are: 1. Mucosa : a. epithelium; b. lamina propria (loose CT with lots of lymphs and lymphoid tissue; muscularis mucosae (a thin layer of smooth muscle). Mucous membrane is synonymous with mucosa or tunica mucosae. So, the mucosa of the GI tract consists of three tissues: the epithelium, the lamina propria, and the muscularis mucosae . (We will see that the mucosa of the respiratory, urinary, and reproductive tracts consists of just the epithelium and lamina propria). Of all the four layers, the mucosa is the most variable in terms of structure and function 2. Submucosa: dense CT containing vessels, nerves (Meissner’s plexus), and lymphoid tissue. There are two places in the GI tract where mucous exocrine glands are present in the submucosa . These two places are the esophagus and the proximal duodenum. All other glands in GI tract are located in the mucosa. 3. Muscularis externa : two layers of smooth muscles, inner layer is circular (i.e. it goes around the tube), outer layer is longitudinal (runs up and down the tube); between these muscle layers is CT with vessels, nerves and neurons (Auerbach’s plexus or myenteric plexus). 4. Serosa/Adventitia: a thin layer of CT covered by simple squamous cells (mesothelium). Within the abdominal cavity, the serosa on each side of the digestive tube fuses together to form the suspensory structure called the mesentery through which blood vessels, lymphatic vessels, and nerves run.
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2 The above describes the general makeup of the entire GI tract but it is most accurate for describing the stomach to the rectum. Why? -because the esophagus and anus have some exceptions. Esophagus: in the proximal esophagus, the muscularis externa is made of skeletal muscle so we can initiate swallowing; in the mid portion of the esophagus, the muscularis externa is composed of both skeletal and smooth ; in the distal esophagus, the muscularis externa is composed of smooth muscle only. Also, the part of the esophagus that is above the diaphragm lacks a serosa. ( Note: the thoracic esophagus and the anus are retroperitoneal structures and hence lie outside the peritoneal cavity (abdominal cavity).
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