Digestion occurs in the gastrointestinal system, which is composed of the gastrointestinal tract (GI tract) and the associated accessory organs.The process of digestion occurs through six steps, namely ingestion, secretion, mixing,digestion, absorption and defecation. (Tortora&Derrickson, 2008).
Anatomy of the digestive system
The GI tract or the alimentary canal is a long winding tube that measures approximately 30 feet (9 meters) in length. The mouth, esophagus, pharynx, stomach, small intestine, large intestine and the anus make up the GI tract.The salivary glands, tongue, teeth, gallbladder, pancreas and the liver make up the associated accessory organs (Tortora&Derrickson, 2008).
The mouth. The mouth starts with the oral orifice and extends up to the opening of the pharynx.The palate forms the roof of the mouth, which helps separating mouth cavity from the nasal passage. The mouth consists of the teeth and the tongue. The teeth help in breaking down the food into smaller pieces, while the tongue helps pushing the food down the esophagus. (Tortora&Derrickson, 2008).
The salivary glands. There are three extrinsic salivary glands in the mouth, namely, the parotid (located beneath the skin of the oral cavity, behind the jaw and anterior to the ear lobe), the submandibular (located along the jaw) and the sublingual glands (located under the tongue).The acidic saliva (pH 6.35-6.85) secreted by these glands is composed of water, salivary α-amylase (ptyalin), chloride ions, buffer (bicarbonate and phosphate), IgA and lysozyme. Saliva breaks down the carbohydrates and the lipids in the mouth (de Almeida et al, 2008).
The pharynx. The pharynx or the throat forms a common passage for food and air. The epiglottis closes the trachea so that food enters only through the esophagus. (Saladin, 2007).
The esophagus. The esophagus is a long tube lined with mucus secreting cells. The masticated food passes down through peristalsis. Upper esophageal sphincter controls the top part of the tube and acts similar to the epiglottis, preventing air from entering the stomach, while the lower esophageal sphincter prevents acid reflux. Esophagus runs through the diaphragm to bulge into a sac like stomach (Saladin, 2007).
The stomach. The stomach is present on the left of the alimentary canal in the form of a bulged J. The smaller curved portion is superior and medial, while the larger curvature is inferior and lateral. The stomach starts with cardiac orifice and ends at the pylorus (opening of the small intestine). There are four regions in the stomach: 1) the cardia, which is a small area within the cardiac orifice; 2) the fundus, which is superior to the cardiac orifice; 3) the body, which is inferior to the cardiac orifice; and 4) the pylorus. The stomach is lined with gastric mucosa, which folds when the stomach is empty. These folds are called the gastric rugae. Gastric mucosa is composed of five different types of cells: stem cells, chief cells, parietal cells, enteroendocrine and mucus cells. (Mills & Shivdasani, 2011).
The liver. The liver is on the right side of the abdominal cavity and is the body’s largest gland. The liver is divided into four lobes called the right lobe, left lobe, quadrate lobe and caudate lobe. The right lobe is larger than the left lobe. There are two hepatic ducts, one for the right hand side and one for the left hand side. These two ducts fuse to form the common hepatic duct, which in turn runs down to carry the bile to the duodenum (Saladin, 2007).
The gallbladder.The gallbladder is located inferior to the right lobe of the liver and has two parts called the fundus and the cervix. The bile from the gall bladder combines with bile from the common hepatic duct via the cystic duct and opens into the duodenum.(Saladin, 2007).
The small intestine. The small intestine is 6 m long and forms the longest part of the alimentary canal. It is divided into the duodenum, jejunum and the ileum. The duodenum starts with the pylorus and is responsible for receiving bile and pancreatic juice. Duodenum ends at the duodenojejunal flexure. The ileum joins the small intestines to the large intestine at the cecum via the ileocaecal junction. Jejunum and ileum help in promoting nutrient absorption. The intestine is lined with villi that take nutrients to the blood and cells.(Saladin, 2007).
The large intestine. There are four regions in the large intestine: the cecum, colon, rectum and the anal canal. The cecum forms the inferior part of the large intestine that ends in the rudimentary appendix. The colon is the major part of the large intestine that is divided into ascending colon, transverse colon and descending colon each of which is at 90 degrees to the other.The descending colon ends in the sigmoid colon. This sigmoid colon forms the rectum. The rectal valves present in the rectum hold the feces while passing gas. The last 3 cm of the rectum is the anal canal that ends at the anus.(Saladin, 2007).
The pancreas.Thepancreasisposterior to the large-curvature of the stomach. Inside the pancreas resides the Islets of Langerhans that produces insulin and glucagon. The exocrine gland of the pancreas produces the pancreatic juice, which is conveyed to the duodenum via the pancreatic duct (Saladin, 2007).
Functions of the different gastric mucosal cells
Parietal (oxyntic) cells. Parietal cells are large and complex hydrochloric acid (HCl) producing gastric cells of the stomach. They are localized to the fundus and upper body of the stomach. The acidic pH causes the proteins in the food to unfold. Protein degrading enzymes act on such unfolded protein thereby breaking them down into smaller molecules.Parietal cells also produce a glycoprotein called intrinsic factor, which is related to erythrocyte maturation.(Lapierre et al, 2007).
Chief (zymogenic) cells. The fundus of the stomach and the gastric glands are the site of the chief cells.These secretethe enzymes pepsinogen and renin, which breakdown the unfolded proteins. (Nam et al, 2010).
Enteroendocrine cells. Enteroendocrine cells secrete the hormone gastrin, which promotes secretion of hydrochloric acid and pepsinogen by parietal cells and chief cells, respectively. Gastrin also initiates gastric contractions, which aid in food-mixing. This hormone is secreted by G cell, a type of enteroendocrine cell.(Sternini, Anselmi & Rozengurt, 2008).
Mucus cells.Mucus cells produce large amounts of mucus to form a lining throughout the stomach. This mucus lining protects the stomach from the activities of the digestive juices. (Saladin, 2007).
Gastric Stem cells. Gastric stem cells differentiate into parietal cells, chief cells and other types of gastric cells to replace the worn out gastric cells. (Modlin, Kidd, Lye & Wright, 2002; Mills &Shivdasani, 2011).
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