Basic anatomy of the gastrointestinal tract

The gastrointestinal tract is a long tube which starts at the oral cavity and finishes at the rectum. This well-controlled tube is made up of many different sections, as well as adjunct accessory organs such as the liver, pancreas, and the gall bladder, each of which have specific functions. The main sections of the gastrointestinal tract include the oesophagus, the stomach, small intestine, and the large intestine. The division of each section is controlled by key junctions along the gastrointestinal tract, followed by a regulating sphincter. The lower esophageal sphincter separates the oesophagus from the stomach. The oesophagus acts as a tube from the oral cavity down into the stomach. The lower esophageal sphincter prevents the stomach contents from coming back up the oesophagus.

The pyloric sphincter separates the stomach from the duodenum, the start of the small intestine. Along the small intestine, there are no key junctions separating its three distinct sections– the duodenum, the jejunum, and the ileum. Instead, there are changes to the epithelial structure, such as the thickness of the intestinal mucosa, the epithelial cell diversity at the apical surface, which is the outermost layer of cells facing the inside of the gastrointestinal tract. Also, the number of villi protruding into the lumen and the number of lymphoid nodule and submucosal glands within the submucosa. These changes become specifically distinct as the small intestine progresses from the duodenum to the ileum.

The ileocecal sphincter and valves separate the end of the small intestine– the ileum– from the beginning of the large intestine, known as the cecum and the ascending colon. Similar to the small intestine, there are no key junctions separating the ascending, transverse, descending, and sigmoid colon sections of the large intestine. Instead, there are changes to the epithelial structure. Finally, the internal and external anal sphincters separate the contents in the large intestine from the outside world. Control of the internal anal sphincter is involuntary, whilst control of the external anal sphincter is voluntary.