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  Digestive system

The gastrointestinal tract or digestive tract is otherwise referred to as the GI tract or the alimentary canal, (nourishment canal) or the gut. The term alimentary canal specifically starts from the oral cavity up to the terminal end of the digestive tract which is the anus while the gastrointestinal tract literally begins from the stomach up to the anus. The digestive system is a collection of different organs that works together to mechanically and chemically breaks down food particles into smaller molecules to extract the energy and nutrients and eventually eliminates the remaining waste. This entire process is otherwise known as digestion.

Digestive system
Digestive system

Digestion begins in the mouth as food, is ingested, chewed, swallowed and eventually moves into the esophagus, a muscular tube that transports food from the oral cavity to the stomach through peristalsis. This is followed by further mixing of the food with the hydrochloric acid secreted by the parietal cells that lines the mucosa of the stomach which eventually forms the chyme, a soupy mixture that exits the stomach. The chyme eventually goes down into the duodenum, the first part of the small intestines where the bile (produced by the liver) and other digestive enzymes from the pancreas (accessory gland of digestion) enters, chemically breaking down the food particles into smaller molecules in preparation for absorption at the jejunum and ileum (the second and last part of the small intestine). Absorption of water and salts happen as the undigested food goes down into the large intestines namely: cecum, ascending colon, transverse colon, descending colon, sigmoid colon, rectum and anal canal. Furthermore, it is at the large intestines where the fecal material consisting of undigested food particles is formed for eventual elimination through the anus. The highlights of digestion includes the following: starch digestion begins at the mouth because of the presence of salivary amylase which is further reinforced by the pancreatic amylase released at the 2nd part of the duodenum; protein digestion starts at the stomach because of the presence of pepsin secreted by the chief cells while fat or lipid digestion starts and ends at the 2nd part of the duodenum due to the presence of bile salts produced by the liver and the pancreatic lipase.

Anatomically, the entire gastrointestinal tract is approximately 7.5 meters long (25 feet) in a normal adult male. It consists of the following structures: mouth, pharynx (throat), esophagus and stomach (the upper gastrointestinal tract) and the small intestines, large intestines and anus (the lower gastrointestinal tract). Related organs considered to be accessory organs that help in digestion are the following: liver, gall bladder and pancreas.

The liver is the largest gland in the human body that secretes bile into the 2nd part of the duodenum (part of the small intestine) via the biliary system. Bile is initially stored at the gallbladder which serves as a reservoir located at the inferior surface of the liver. Bile is released when the gall bladder contracts upon cholecystokinin (CCK) stimulation coming from the duodenal cells. Another accessory gland is the pancreas, a retroperitoneal gland that secretes an alkaline fluid containing bicarbonate and enzymes which includes trypsin, chymotrypsin, lipase, and pancreatic amylase, as well as nucleolytic enzymes (deoxyribonuclease and ribonuclease), into the small intestine.

The gastrointestinal tract also contributes significantly in our body’s non-specific defense mechanisms in the immune system. The stomach’s ph ranges from pH 1 to 4 which is fatal for many microorganisms that enter it. Furthermore, the mucus contains IgA antibodies that fights off these microorganisms which protects us from getting sick. Other non-specific defense mechanisms of our immune system along the GI tract includes enzymes in the saliva, bile and health enhancing boosters, the colonic resident bacteria.

The GI tract has a general histologic feature but with some slight differences due to some particular specialized areas of function. Typically, the GI tract can be divided into 4 concentric layers: mucosa, submucosa, muscularis externa (the external muscle layer), adventitia or serosa. The mucosa can be further subdivided into lining epithelium, lamina propia and muscularis mucosae. It is the innermost layer of the GI tract, surrounding the lumen of the GI wall which comes in direct contact with the food and is responsible for absorption and secretion process of digestion. Along the small intestines, the mucosa has unique features namely, villi and microvilli which are folds to increase the surface area for absorption. In the stomach, the mucosa consists of gastric pits which are invaginations of secretory gastric glands. The second layer, submucosa is a dense irregular layer of connective tissue where the mucosa derives its blood supply. It consists of large blood vessels, lymphatics and nerves like Meissner’s plexus branching out towards the mucosa and muscularis. The third layer, muscularis externa is composed of two layers of muscles namely: a circular inner muscular layer and a longitudinal outer muscular layer. These smooth muscles particularly the circular muscles are responsible for peristalsis, coordinated contractions that propel the food downwards toward the anus and thus, prevent the food from going backwards. The longitudinal muscles shorten the GI tract when it contracts. Found in between the two muscle layers are the myenteric or Auerbach's plexa. The outermost layer of the GI tract is the adventitia or serosa which consists of several layers of connective tissue.

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