The Digestive System
The organs of the digestive system can be separated into two main groups: those forming the alimentary canal (the gastrointestinal tract), and the accessory digestive organs. The alimentary canal digests and absorbs food. The accessory tract assists the alimentary tract.
The alimentary canal is a continuous, coiled, hollow, muscular tube that winds through the ventral body cavity. It's open to the environment at both ends. Its organs are the mouth, pharynx, esophagus, stomach, small intestine, and large intestine.
Mouth
Food enters the digestive tract through the mouth. The labia (lips) protect its anterior opening, the cheeks form its lateral walls, the hard palate forms its anterior roof, and the soft palate forms its posterior roof. The space between the lips and cheeks externally and the teeth and gums internally is the vestibule. The area contained by the oral cavity is the oral cavity proper. The tongue occupies the floor of the mouth and has many bony attachments. As food enters the mouth, it is mixed with saliva and masticated (chewed). This is where the breaking down of food begins.
Pharynx
From the mouth, food passes posteriorly into the pharynx, which is the common path for food, fluids, and air. The pharynx is divided into three sections: the nasopharynx (air from the nose passes here), the oropharynx (food and air from the mouth passes here), and the laryngopharynx (air going to the lungs passes here).The walls of the pharynx contain two skeletal muscle layers. The cells of the inner layer run longitudinally; those of the outer layer (the constrictor muscles) run around the wall in a circular fashion. Alternating contractions of these muscle layers propells food through the pharynx into the esophagus below. This propelling mechanism is called peristalsis.
Esophagus
The esophagus runs from the pharynx through the diaphram to the stomach. The esophagus conducts food to the stomach by peristalsis. Begining with the esophagus, the walls of the GI tract have a basic pattern that reflects their common functions. Because the tissue arrangement in the alimentary canal walls is modified along its length to serve special functions, here are the basic wall functions for reference.
The walls of the alimentary canal organs from the esophagus to the large intestine have four characteristic layers:
1- The mucosa is the innermost layer. Its a moist membrane that lines the cavity or lumen of the organ. It consists primarily of a surface epithelium, plus small amounts of connective tissue, and a scanty smooth muscle layer.
2- The submucosa is just beneath the mucosa. It is a soft connective tissue layer containing blood vessels, nerve endings, and lymphatic vessels.
3- The mascularis externa is muscle layer made up of a circular inner layer and a longitudinal outer layer of smooth muscle cells.
4- The serosa is the outermost layer of the wall. It consists of a single layer of flat serous fluid-producing cells, the visceral peitoneum.
All layers of the alimentary canal wall except the mucosa contain a nerve plexus, and intrinsic network of nerve fibers that is actually part of the autonomic nervous system. These plexuses help to regulate the mobility of the GI tract organs
Stomach
The c-shaped stomach is on the left side of the abdominal cavity, nearly hidden by the liver and diaphram. The stomach has four different sections.
1) The cardiac region surrounds the cardioesophageal sphincter through which food enters the stomach from the esophagus.
2) The fundus is the expanded part of the stomach lateral to the cardiac region.
3) The body is the midportion, and
4) The pylorus is the terminal part of the stomach.
Digestion occurs here mechanically, as the muscular stomach crushes food, and chemically, as hydrochloric acid and other gastric juices and enzymes break down food.
Intestines and Other Digestive Organs
The majority of chemical breakdown of food and absorption into the body occurs in the small intestine. It contains many different enzymes to break down a variety of food components. Microvilli (small fingerlike projections of the endothelium) provide a large amount of surface area through which to absorb the broken-down nutrients. Some chemicals are also provided by other organs in the body. The pancreas provides the small intestine with an enzyme called pancreatic amylase, which breaks down starch. The liver produces a substance called bile, which serves to emulsify, or break down, fats.
After being processed by the small intestine, food is transported by peristalsis into the large intestine throught the ileocecal sphincter. The large intestine does not actively break down food, but absorbs as much of the remaining water in it as possible. It also contains millions of bacteria which metabolize any remaining nutrients. The large intestine provides the final opportunity for absorption in the GI tract. After food is processed here, the remaining waste is expelled from the body through the process of defecation.
written by Michael Simpson
